So it's doable then - in fact, quicker and cheaper than the UK setting up it's own competitor GNSS. I'd also look very carefully at what UK supplied means. Do you really think the relevant data is only physically located within the UK, given the multinational nature of the company doing the work? That individuals can't change jobs and do the same work for a different employer? Details again - but no change there, lack of attention to detail is what has killed Brexit so far.
Let me help you - no one doing the crypto is going to jack in and go work for the French.
The EU will have to generate the capability themselves, and even then, as the 5 Eyes have demonstrated, much to the regular fury of the French, we can break anything they've developed to date without breaking a sweat.
Now as to this 'quicker and cheaper'… Galileo is late, its last generations technology, its got technical flaws, its not fully functional and may never be the dream they thought it would be. The EU is already considering tenders for a replacement constellation. So, we'd both hit the ground at the same time, and we have better software and a global network of ground stations the EU doesn't.