Hi Airbornestu,
I have done quite a few Channel crossings, but most of them were in the South East of England Lydd - Le Touquet was a really common route for me (partially because I am based out the south east, and partially because I have this fear that you're talking about).
The way I look at it is, I will fly as high as I can (within reason) in order to limit the unlikely event of having to ditch (by having greater glide distance). Or that should I have to ditch, I would endeavour to have as much time as I can to at least attempt an engine restart / change tanks / call mayday and find a big ship nearby to get them to pick me up (and maybe prepare a couple of margueritas for my arrival). When I cross the Channel by the Isle of Wight to over Cherbourg, I try to climb to 10,000' because I've calculated that my risks were substantially lower up high than they were down low. [probably overkill looking back although the cloud tops were around 9,500' so I guess it worked out pretty well!].
A friend of mine does the long crossings at 4 or 5000' and say "Well, that still gives me plenty of time to restart". He has done numerous crossings into IoM and over into Ireland from there too, although I understand that he climbs to 10k - 14k on a regular basis to do so.
I've never been the Isles of Scilly, but that's definitely on my list! Hopefully soon!
Good luck with the night & IMC ratings! They've boosted my confidence dramatically, I hope they will boost yours too!