In addition to the items mentioned above....
I used to have a "trade" source for distress flares, both acquiring and disposing of them.
Both have now dried up, and conventional pyrotechnics have become a right pita, on both respects.
So, I've recently bought an OREO LED "laser flare", and was so impressed by its performance, that i bought another!
I have the bigger version mounted on the cabin bulkhead, the smaller fits in a LJ/BA pocket. About £75?
If you carry nav lights, it's probably prudent to carry a visual distress signal for use in darkness? These flares are programmed to send a red SOS signal, as well as flickering to simulate a flame/conventional flare.
Good reviews in PBO etc.
I would also highly recommend a GMDSS Personal Locator Beacon PLB in addition to the VHF/mobile. £200?
In fairness, I guess the DSC VHF might fulfill a similar function?
However, here in Cornwall and, for example in the W Highlands, VHF coverage can be patchy. COSPAS-SARSAT GMDSS is supposed to solve this.
I'm sure you'll be OK in the Solent, or thereabouts...
I'm also slightly paranoid about bilges, and so have fiitted an additional (double-acting) bilge pump.