Choosing a new Carbon bike £1500 ish

I was advised to get a defy for endurance paced riding. 55y of age, historic back problems. It was fine. Incident with a a car meant I needed a bike quickly and got a tcr disc. Different racey geometry but wow, what a difference. Nimble yet stable and a joy to ride. Also comfortable. Works for me even on a recent 130mile double devil ride. Unless you are sold on the upright ride of the defy I would have a look at the tcr 

I went down the route of buying direct from a team sponsor, specifically the UK distributor for the brand (KTM) and the bikes - some were built up with DA9000 & frames they had been designated for the team to use, these were the 'leftovers'.

I got a fully warranted frameset for £430 posted and they sold me a brand new DA front mech for £20 as well. I built it up from there with exactly what I wanted and though a few years ago (2013 machine bought in 2014) I managed to get full Dura Ace 9000 barring  very lightly used carbon FSA chainset, branded carbon wheels, new Ritchey Superlogic post, new Selle Italia carbon railed saddle and barely used top end Modolo carbon bars for a total of just over £1500.

I've seen team sponsors and indeed bike shops sell their excess stock and display only bikes often on ebay with the same warranty as you'd get normally. There's also the private option where someone has literally had a spin round the block or a couple of short rides and found cycling isn't for them, those are when you can get bikes for silly low prices but obviously it's difficult to transfer the warranty if at all. 

As an example, there's a Cinelli Superstar with 8020 hydraulics + uprated wheels for £1400 and a 100 miles on the clock on ebay right now. These are the kind of bargains you can find regularly, especially if they are a couple of seasons old. I saw last year a new ex-display Corratec full carbon bike with 10 speed 105, from a shop and it sold for £595 on a Buy it now but had been for sale several months.

Luck finding something.

I must say, the Planet X Pro Carbon Evo with Force 22 hydro for £1,200 is a bargain. 

I'm a big fan of Force 22, it's basically Red but a touch heavier, it all functions the same. 

The only thing that needs changing is the wire bead tyres and there's £300 left to do that with. You'd then have a super fast and pretty damn light bike that'd happily thrash things twice the price. 

Only issue is that it doesn't have a sexy brand name on the downtube. 

Just remember that "same spec" does not neccesarily mean same spec. I'm not sure on the year of the Giant for that price but the cranks and brakes will not be 105. Not saying that they are better or worse the the 105 specific items and sometimes it is because of BB spec for the bike but something to be aware of if you were looking to upgrade items. 

   If you have nothing against mail order then Canyon have the Endurace CF SL disc at £1700 plus postage. Full 105 groupset,Aksium Elite disc wheels with 28mm tyres. Top spec for the money.

   How about something non carbon like a Cannondale Cad 12 disc. Never ridden one but the consensus is that its a great frameset. Wheels look a bit basic but they are usually at the top of the list for upgrades.

   I have 2 good quality carbon bikes and a higher end aluminium and i can detect very little difference in ride quality but the aluminium bike is a tad heavier. Also having had a bike with disc brakes ,105, for me,i can't see the need for them. If you're commuting every day in winter then maybe they have an advantage but for recreational rides i don't see the advantage for the extra cost.

I recently went through a similar process and ended up getting stuck on a Giant (TCR in my case - preferred its looks to the defy) for many of the reasons you mentioned - mostly value and lbs availability. I wasn't really excited by the idea of a giant, but it just seemed the obvious choice. Very glad I got it so far though.
So in answer to your question, obviously I can't see a reason not to get one either..