So it really comes down to a personal choice of the level of risk you are prepared to take.
Agree with Pace here. It's the level of risk you are willing to take, and the mitigation measures you take to prevent bad things from happening or from escalating. If you're not prepared to take any risk, you should not be flying privately, in a single engine aircraft in the first place.
Read the Equipped to Survive link that Mark sent you. Not just that link, but the entire site. It's a goldmine of information. Then, ideally, go on a maritime survival course to actually learn how to operate all the equipment, and to feel what it's like bobbing about in a hostile sea, having to operate survival equipment with numb hands and such. Then read some of the other threads on here about crossing the channel and such. And then decide for yourself what sort of gear you're going to bring, assuming you're still willing to take the risk in the first place.
Me personally, I have a drysuit and life jacket but no life raft for crossing the North Sea. Plus some other survival kit: rescuestreamer, signal mirror and a bag of tricks I picked up at the survival course.
I see a lot of people crossing vast expanses of water with just a life raft and some life jackets. After having done that survival course, I know that combination gives a very false sense of security. After the ditching itself is over, hypothermia is your biggest enemy. A dry suit does a lot more to prevent hypothermia than a life raft. And that's assuming you can get the life raft deployed properly, turned right side up, and are able to get into it in the first place. Which are major issues all by themselves, and for which you need proper training.
Furthermore, a 10+ kg life raft is a liability in itself in the event of a ditching. It needs to be secured properly otherwise the impact may launch it forward and hit you in the head or something, but it also should not be secured too well as you may only have a few seconds to retrieve it before the aircraft sinks.
At the end of the day, the only things that are guaranteed to make it out of the aircraft together with you, are the things you carry on your body.