I agree with Whirly - who needs navaids? Just point in the right direction, and you're bound to hit land somewhere! (Some kind of navaid as check that you're going to hit the right bit of land is nice, but not essential, since it's usually pretty easy to identify a piece of coastline and correct as necessary.)
More important, I would say, is some instrument training - although it's not actually required. I've flown to the Channel Islands on a very hazy day, and although technically it was VMC, I was pretty much flying on instruments the whole way (made even more fun by the fact that the AI wasn't working, so it was partial panel, but that's a different story).
I don't think I'd go any kind of distance over water without life-jackets. A life-raft is nice if you've got one available, and got the weight for it. Neither of these is actually required, though.
Pooleys contains just about everything you need to know about customs, special branch, flight plans, etc, but it is spread through several different sections so you'll need to read it pretty carefully to make sure you don't miss anything. Also worth a read is the CAA Safety Sense leaflet on VFR flight plans.
FFF
-------------
Edit to reply to Penguina - some clubs hire them out (White Waltham, for example, have one which you can hire as long as you book it before anyone else - which can be tough in the summer). Otherwise, there's a company called South Eastern Marine Service which will courier one to you anywhere in the UK for short-term loans. Don't have the details to hand, but the address and phone number is in the back of one of the CAA Safety Sense leaflets, which is on-line somewhere if only I could find it! (Thanks to capt_sparky for finding this information for me a few months ago when I needed it.)