Let's assume the risk of Santander UK defaulting is very close to zero. Let's also assume the Bank of England is not going broke any time soon. So our default risk is effectively zero.
As the other answer notes, there is no big difference between where the GBP is held. If you're looking for a way to transfer GBP to EUR, I like Transferwise, but you might be able to have your bank in France give you a great rate after transferring the GBP over to them. Or they might price gouge you on the exchange rate. Be careful. In order to minimize the inter-bank transfer costs, minimize the number of times you make this type of transfer. You will get hit once if you do it, so try and keep it at a minimum.
The real risk here is currency devaluation, as well as inflation risk. If your GBP depreciates significantly vs. the Euro, your purchasing power will go down, all else equal. If you leave your money in a checking account and earn less than the inflation rate, once again your purchasing power will go down as it is worth less in the future.
I am in an extremely similar situation myself, although with GBP and USD. The bottom line is that you need to answer the question, "Where does it make the most sense for your money to be? Which country and currency? What makes the most sense for you?"
After you decide what you are saving for, and which currency the funds should be in, you'll want to think about how to invest that money to earn a higher return. Here is a very basic overview on investing. Before you begin speculating on the long-term health of GBP vs. EUR, you should seek to gain a very modest return by investing in something relatively low-risk, and appropriate for your time horizon, such as money-market funds or low cost ETFs.
DISCLAIMER: If you have any doubts as to the merits of what I have said you should seek advice from an independent financial advisor.