Then jump on an easyJet/BA/Ryanair flight to Heathrow, Gatwick, Stansted, East Midlands, Birmingham, etc and do a Class 2 medical with one of the EASA AMEs based on the airport.
Getting a medical in France needn't be an issue at all, the benefits of EASA are that you can use any EASA AME. They send the report to France and give you a certificate.
Forgive me for being a little direct, but getting a validation seems like an expensive and long winded way of achieving what you want when you already hold a French EASA licence. The CAA will charge £46 to verify your licence details, and the DGAC about €80 to issue the validation. At the rate the CAA are processing paperwork, it could be a month before they talk to DGAC at all.
Source:
EASA Medical FAQ
Good luck with whatever path suits your personal circumstances..