TCR, Defy or Gravel Bike?

Tiagra 4700 vs 105 5800? Performance is identical. You're just giving up the 11th cog and a bit of weight on Tiagra, which is huge credit to this underappreciated groupset. Shifting is leagues better from the older Tiagra 4600/105 5700 which were the same-generation contemporaries.

Knobby 28 mm tires? I honestly don't know of any. Panaracer Gravel Kings are the closest I can think of, but they barely count as such. If you're keeping the Defy, you will have better luck "winterizing" it with tubeless in the same width, I think. A friend of mine has a 2016 Defy Advanced 2 and he struggles fitting anything wider than a 28 mm Continental (to be fair, their tires tend to mount +2 mm wider than stated).

Gravel/2x cyclocross bike is much more versatile, but metal versions also tend to carry a bit of a weight penalty, much of it in the (overbuilt?) wheels. 

I've just taken delivery of a carbon Defy (Advanced Pro 1) and it is very nice

It's certainly faster on the climbs than my old bike (steel framed Kona Rove). Part of it is the 5kg weight difference and part is the stiffness due to the oversized BB - you can really feel it wanting to accelerate.  I didn't try the TCR but I'd be surprised if it was much different apart from the geometry.

I have a TCR Advanced as my nice bike, and it's great. Comfortable despite being very stiff and responsive. They are quite long so check the sizing if you are of the short torso persuasion. I ride a medium though height wise I should probably ride a M/L. When I go home to visit the folks I borrow a friend's aluminium Defy 1 and the difference is noticeable. The defy feels slower, particularly on the climbs. It's not just the weight, it just doesn't respond like the TCR does. Whether that translates into a speed difference I couldn't tell you but given the choice I would ride the TCR every time although the defy is certainly a nice bike. A carbon defy might be a different beast however.

Another tcr owner here. Never ridden a defy but I came from a more 'sportive' geometry bike (kinesis) which is now relegated to winter duties.

The tcr definitely encourages you to press on. It's high stiffness and low weight encourages you to attack hills (at least until you blow! ). You may not be much faster but you might have more fun on it. Don't find it uncomfortable, have done 100+ mile rides and I actually find it very smooth, but you can put pretty wide tyres on it if need be.

Regarding the Tiagra vs 105 question..  they are both good groupsets the main difference is that Tiagra is 10 speed and 105 is 11 speed. I've ridden with both and I'd be happy with either... a lower groupset is easily upgraded at a later date.. 

I've had/still got an older tcr and defy non disc aluminium. TCR was fine for longer distance (100 miles) though I found it a bit less comfy that defy - more stretched out. TCR always felt faster but in reality looking at strava segments there's very little in it. Fitted guards to the defy and use it as my crap weather bike. Despite being heavier I still enjoy riding it. You can spend hours agonising over choices but in reality I'm not sure it makes a whole heap of difference...

Have you been been training with power on your Smart Trainer?  Is your FTP where you want it or could you improve it over winter with a structured training plan?  If you have reached peak Wattage I'd go for the power meter to scientifically  translate your turbo gains onto the road but otherwise I'd make sure I'd made the most of the turbo trainer first and get the PM another day - they're only going to become better/cheaper.

Disclaimer:  I've just bought a PM for my new Defy so I'm in no position to lecture

Divorce your wife and get whatever bike you like.

I'd suggest if you are committed to training G it's not the power meter that's doing much other than telling you a start and end number. Taking the weight analogy, you can go to the rack and pick up a 3kg weight for a few weeks then a 5kg and onwards, if you pick up a 20kg surely you know it is wrong. HR, Cadence, Speed, FTP are all good indicators, maybe not so instant as a power meter but much more cost effective.

I'm on my second TCR now..  If you can do 20mph on your own for 30miles (as per your TT record), then I'd suggest that slamming your stem and getting a bit more aero would be the most efficient way of going faster if that's what you want to do.

The stock wheels on everything up to the top level giant bikes are dogshite, you'll get an instant hit upgrading those.

I'd ditch the power meter idea, you'll have far more fun with a better bike/parts upgrade than you would if you pay money to constantly felt anxious about your power output... I think the advice above to get a PM before a better bike is ludicrous. I train on HR which costs £50 for a strap, after a while you can just feel what's going on accurately enough. Now I only use my HR monitor to ratify what I already know OR if I'm doing a roller session. I'd like a PM don't get me wrong but it's a 'luxury item'.. I already know I struggle to keep up with my mates on a climb, and I monitor my HR for TT's.

However saying that, you can very cheaply put a set of drop bars on your hoy (£20) and you'll instantly have your gravel/endurance bike right there.. so getting a racy TCR and owning a do it all gravel/commute is closer to covering all the bases in my humble opinion. 

I ride my TCR wherever and over whatever most of the time (tubeless tyres), I have no issues with comfort, however I can 'see' where I'd be a bit more comfortable if I was a little more upright (defy style).  But that comfort would be offset by being a bit slower (less aero). 

Compromises at every turn..  I don't envy your decision.

TCR compared to my old aluminium bike saved me around 20 seoncds up a 5min climb for same power. SO not much in it
Power meter literally saved me minutes up the same climb as i went from 4w/kg FTP to 5w/kg FTP in about 1 year. Power meter helps you even if you dont do much training as it helps you pace so much better. If i were you , definitely get a power meter first, then save and try and get a tcr/nicer bike. But power meters are such an incredible tool long term.
You wouldnt go to the gym and pick up random weights, you would build up slowly looking at the masses of the weights. Its the same with a power meter, you can finally see what effort you are putting in, not just blindly guessing on feel.

I kind of get what you are saying. I did similar, and flogged everything and bought a gt grade. I use it as my road bike knowing that it is 95% as quick as an actual roadbike, but now i can basically go everywhere.

Not ridden them but if you can get some studded 28's or even tubeless 28's to run at lower pressures then the current posh could certainly be winterised - the (C2W plus Hoy sale) may even allow for summer and winter wheels on this. With the right tyres, pressures, mudguards you don't really have to go to 32's for comfort.

Then look at Carbon for your posh bike, we keep seeming to be recommending Cannondale's at around the 1k mark - 105 or above is easily within the budget.

Power meters - meh! Everyone i know who has one is far less talented than the price and benefit dictates. We surely have enough information at our digital fingertips to know how hard we're working and if we're improving (and the current indoor will allow you to do so). Otherwise your legs will know Especially at the price brackets you're talking, why spend half the value of your bike (plus) on something to tell you how you're riding it?

As for the naughtiness, cyclescheme seems to now even offer an accessories only option so really not too naughty. I'm deciding between a Smart Trainer and wheel upgrade for the summer bike and the Cannondal Slate Apex (just because it looks cool)...

Sell the city bike. Get a gravel/endurance/cx bike (e.g GT grade). It will open up a whole new world of cycling opportunities. There's some fantastic riding when the tramac finishes...

I can't decide either.  Hmmm...

Sell the Defy and do all your road riding on the TCR. But then you don't have a winter bike for quick, lengthy road rides. 

How about option 2. and get rid of the Hoy? 'Winterise' your Defy - including fitting cyclocross tyres for gravel-ish rides? The  position on a TCR is not so extreme. When I'm on my winter bike I can struggle to keep up with my younger ride buddies on their Defys. When the sun is out so too is my TCR composite and the playing field is levelled. 

Buy the best TCR Advanced you can get and try to recoup some money by flogging the Hoy to spend on other goodies. But don't waste that money on a power meter.