Nothing new to add to what´s been said already but just to reaffirm the following points:
* Manual lifejacket always, as thought on HUET courses.
* A lifejacket by itself is only useful to help with recovery of the bodies (I'm not being sarcastic).
* An immersion suit means the concerned body will hopefully still be warmish by the time it's recovered.
* A liferaft means the warmish body will also likely be conscious (if a bit groggy from the sea sickness pills) and will be so much easier to spot from the air.
* For the odd cross-channel trip, you may accept the small risk involved and just chance it without a suit and a liferaft. On the other hand, if you are planning a series of crossings, I would get both and brief the whole plane load on how to use it.