SEMS (South East Marine Services) will arrange dingy practice etc in a local pool if you can get enough folk together. Did it a few years ago, and if it taught me anything it was that getting into a dingy in the non threatening environment of a swimming pool was hard work. I hate to think what it would be like in the Channel.
Andark (nr Southampton) and Fleetwood (nr Blackpool) both offer aviation related survival courses involving the use of dunkers. Fleetwood have a light aircraft style cockpit they can dunk you with, Andark I think use a heli type.
I hope to be doing an Andark course next month which will include liferaft and life jacket skills as well as being dunked a few times.
Both require minimum numbers for the courses (6 or thereabouts) and the cost is around the £150 mark each. I'm sure if someone were to take the lead you might be able to get a group of PPRuNer's together to go on one.
SEMS also offer reconditioned immersion suits at either £100 or £115 depending on the type. I bought one a few weeks ago and am very impressed. Only used it once so far on a reasonably warm day. It wasn't too uncomfortable on the ground, fabric is Gore-Tex, and once in the air was fine. You only need to put it on just before you get in to start up so you are not wearing it for too long prior to getting airborne.
A friend who makes regular trips to France has been using one for a year or two now with no problems. When he damaged one of the wrist seals SEMS fixed it very quickly.
If you want to see a copy of the SEMS brochure for the immersion suits e-mail me with a fax number.
WF.