Good information in the CAA document.
If traveling with others, I'd seriously consider meeting them on the other side of the Channel. You can then keep the liferaft on the front seat.
Boating and paddlesport suppliers may offer drysuits at a better price than aviation products.
Rafts are highly variable in quality. Good ones are expensive.
EQUIPPED TO SURVIVE (tm) - Ditching an Aircraft Note that most of the stories there are warm water ditchings.