Welcome to the forum.
As you are living in the UK, and plan on
purchasing a
boat in
France or
Italy and sailing in those waters to start, the UK issued
ICC (International Certificate of Competence) is probably the best way to go. It can be attained through the RYA. Here's a link for more info on the
ICC and other
boating abroad topics:
https://www.rya.org.uk/knowledge/abroad/icc
You don't say whether you already have experience, or even what type of
boat you plan to buy... so hard to give any more
advice, but be aware that there are different requirements for the ICC depending on Sail or
Power, including the size, and whether you plan to sail Inland Waters.
If you have sufficient and documented experience, and can provide evidence, you can challenge the ICC exam. There is a fee.
If you have no experience then probably the easiest way to get some would be to take a course such as the full RYA Day
Skipper course. This includes both classroom and
water based instruction and at the end you (assuming you pass) you will receive an RYA certificate as well as automatically be eligible to apply for the ICC for free, and the instructor/school should help you with all the relevant paperwork.
If you maintain your RYA
membership then you will benefit from free renewals of your ICC every 5 years.
Good luck!