Always nice to help a fellow MCR man.
You will need to clear customs inbound to the UK. Bournemouth and Southampton are possibilities, but you are in a corner of England I do not know very well so others may have a quieter option.
I do not know GE0101, but you could give them a call, I am sure you would get permission.
The return flight is the same.
XPDR is not a necessary in controlled airspace, but it will help. The flight plan for an internal flight into controlled airspace is done on the radio, no need to file like you would to cross and international boundary. Military are usually accommodating, but you can go over or round. The rules are not too different to France, so in practice you should not have a problem.
Good luck with the ais web site, most of us cannot get it to work at the moment!
Rod1