Life Rafts etc
Speaking as a cross-channel yachtie ( I reckon the kit on offer to you will be similar or same, but even more expensive than ' Marine' if an 'Aero' tag can be applied ! ).
Most rafts start at the 4-person size.
Beware having too large a raft for your max crew complement, as it will be less stable in rough seas, ie too bouyant/ light.
The £500 jobs are basic, with a single skin floor - for longer trips where one might be in the raft a long time, a doubled, thermal insulated floor is worth it's weight in gold but I'd think unnecessary for you.
Things to look for are a decent boarding step, as much reflective tape as possible, and the better ones have a light on top, inside & out.
Normally servicing is required every 3 years, about £100.
I'd check on any aero regs', & then if poss' shop around yacht chandlery ad's in the magazines, looking at each manufacturer's spec' on their websites.
As for life-jackets, there are some very neat and unobtrusive high-spec' jobs around now - check Lewmar for instance - but I wouldn't think auto-inflation a good idea in a ditching, as it may inflate & trap one inside; I'd go for 'manual gas'. These also need servicing every few years but the time period is not as strictly stated as rafts, just common sense really.
Thigh straps and spume visors are becoming standard, which is completely right.
Flares are going out of fashion on yachts, as they're nasty dangerous things at the best of times, and there was an horrific accident a little while ago when a qualified instructor, demonstrating a flare correctly, had it fire through the base and fill his abdomen with phosphorous - only just survived, and I think sadly now handicapped.
A design fault with those particular flares recalled ( I had some ! ) but a nasty wake-up call.
I'd still carry a few basic flares, but a waterproof hand-held VHF ( about £150 + ) would be very useful.
Remember that will be useless with the noise of a rescue helicopter nearby.
The Royal Yachting Association run short sea survival courses, with the chance to try a raft in a pool, & training. Not expensive & well worth it, particularly for peace of mind of nervous passengers.
Hope that's some help, and that you NEVER need it !