Well I haven't "ditched" a light aircraft but for other reasons I have some insight into the subject......
I spent some of my formative years spent in an environment where sea survival skills was regularly taught, practiced and sometimes had to be used in anger
. I'm glad to see your not underestimating the sea around the UK because it can at best render you immobile and unable to function very quickly and at worse kill you very quickly if you're not prepared.
Personally, no matter how "infra-dig" it seems to you or your wife I really would second/third/forth all the previous good advice you've had to
wear some sort of survival suit and a lifejacket, and carry on it/in it, some sort of personal locator beacon. A life raft is a "nice to have", but as has been said you may not have time to get it out of the aircraft if you ditch, and if even you do you may not be able to board it, so invest in one only after you've got survival suits/lifejackets/PLBs.
Also be aware in a ditching (or similar) once you've hit the water and stopped all you've done is swopped one problem, surviving the landing, for another equally serious one, surviving the water - any training you can get will markedly increase your chances of survival............
Edited to add: Extra information here:
Hypothermia Prevention: Survial in Cold Water | Minnesota Sea Grant
It's worth bearing in mind for much of the year the sea temperature around the UK is below 10 celsius.