One big UK sportive

If you don't get into the FW this time round then you can always do it on your own/with mates at any other time with the Four Seasons challenge:

http://www.fredwhittonchallenge.org.uk/fourseasons.php

Next year I'll definitely be doing the Dartmoor Classic (£33) and the Exmouth Exodus (£free). Both are fantastic events. This year the Exmouth Exodus in particular was the best night I've ever spent on a bike.

Rather than offer my opinion on sportives and where to ride etc I will just answer your question.
If you want the toughest ride in the UK I'd go for the Fred Whitton (if you can get in!). Stunning scenery, mostly short but exceptionally steep climbs and a great day out. The organisational and local cooperation is fantastic giving the ride a much smaller feel than rides like the Dragon or Etape Cymru/Caledonia which are very 'professional' in their organisation with a price tag to match!!
For the Fred think loads of Malt Bread covered in jam rather than energy bars!
I'd also look at the etape du Dales which will cover a lot of the roads/area of stage 1 of the TdF route. Another very tough day on the bike.
Whatever you choose (or don't) enjoy!

I'm in the ballot for next year's "Ride London 100", but it's the only one I'll be doing, apart from the London to Brighton ride, which I've been doing since its inception. What appeals to me about the "Ride London" is the closed roads and the sheer number of riders. It makes me feel like I'm riding the Tour de France, which is pretty cool when you're a fat bloke with dodgy knees.

But if I want to go for a ride with friends, on public roads and mixing with traffic, I don't feel a compelling urge to pay a commercial company to "manage" my odyssey. My satnav app keeps me on the correct route, pubs provide, er, refreshment, Strava makes a "timing chip" superfluous, my wife's home made meat pie and chocolate cake trump the saccharine hit of a gel, if I want a goody bag I'm sure we could arrange that amongst ourselves too, and, occasionally, our cycle tracks will become entwined with other groups of riders out on the road, as is the nature of such perambulations.

In fact, if "critical mass" participants can harness the power of social networks, to enable everyone to meet up at a certain place, at a certain time, then it's not beyond the realms of credibility for "mamils" to do the same, and book a venue with the money they save to have the mother of all post event "blow outs".

So, in essence, my ride experience is little different to that of a typical UK sportive, except that it's free, and far less likely to antagonise the local populace.

I'll only pay if the ride offers a "closed road, pseudo Tour" experience.

Of those you've listed in the UK, the only one I've done is Etape Caledonia, which is great fun... but the roads are so quiet around there that you don't really get the benefit from the closed roads that you do in, say, the London Ride100.
Assuming you're a relatively experienced cyclist, the only hard thing about Caledonia is the potential for bad weather. The second time I did it, there was a 30 mph headwind for half the route. Ouch.

Go and do the Ardechoise. you can probably drive down, stay over and ride for what it costs to do the Dragon these days

it's the most enjoyable event i've ever participated in. they basically close a whole county.

http://www.ardechoise.com/site/

Why pay to ride on these roads with your mates? Why dont you arrange a weekend away somewhere, get a few good rides in and save the entry fee? If you want to ride with a number on, enter a race, if you want a social ride, do it for free with a few mates and make it an adventure, jump on a ferry to Calais (£20 each way) and ride down to see a stage of the Tour or one of the classics, stay over night and ride home the following day. As you may have guesses, i am not a fan of sprotives, paying to ride on public roads is not my idea of a good day out. Do you realise that there are 3 stage starts and two stage finishes within 60 or so miles of Calais this year? We are going over on tuesday, catching the race at 3 places and coming home on thursday, staying in a chalet just outside Ypres all for 2 nights and getting in about 16 miles in 3 days. The ferry and chalet is going to cost us about £65 per person, not much more than the entry fee for riding around in Britain on public roads. No brainer as far as i am concerned.

I've lost my UK sportiv mojo these last couple of years, maybe it's just me, but...
I'd suggest La Marmotte for a guaranteed Epic and the 2014 Roubaix Challenge, which is biannual (alternating with a longer, if lower-key version) and widescreen, with Mavic support, gendarms on motos with fat knobbly tyres, closed roads, and finish in the velodrome at Roubaix. We even got to clean off in those famous showers (I chose Coppi's cubicle).