A great thread - Cat Driver has nothing but spot-on contributions to make (sounds like from the experience). Here in Scotland one doesn't have to go too far from Prestwick, Glasgow or Cumbernauld before the forced-landing choice is mountains or water, with nothing in between.
I went on an RAF Kinloss PPL survival course a couple of years ago (written-up by James Allan for Pilot), where the bottom-line advice was, if the options were ditch close to a beach or crumple the aeroplane on a cliff-edge, then go for the latter. Having been in Scottish coastal waters in summer, I think I'd try for a mountainside or cliff-top every time.
Go for a water survival time of a couple of hours, max, then see how far offshore you could ditch before a Sea King could get to you (say 90 knots against a prevailing Westerly, plus scramble time). I've done it for the West of Scotland and I think that any further west than Arran or north of Islay means that the only use for a lifejacket will be for the rescue guys to find something to give to my wife for burial!
Be interesting to find out what any RAF/RN/Coastguard crews think.
I also recollect a letter in Pilot from an RN SAR pilot who said that the Coastguard have a VHF frequency, which might be useful, but I can't remember what it was - I suppose one day I really should get round to 'phoning them...