From the information you have provided, I believe that we may be able to help, however we would need more information such as full details of income and expenditure and when you plan to go ahead with the transaction. You would need to arrange a residential mortgage to buy the property in France, with the ability to be able to change this to an expat buy-to-let mortgage when you move. The time frame of this transaction is also important and I would need to know exactly when you are planning to retire and whether the property in France would be a holiday home first before you live in it full time.
When we have this information we could then approach a lender that would allow the release of funds for purchasing a home in France and then allow it to switch onto a buy-to-let basis at a later stage. Based on current criteria this would be possible, however, I cannot comment on whether this would still be the case when you move.
I consider that the best course of action would be to contact one of brokers to discuss the options more fully. Once we have a full profile of your circumstances we can decide which lenders to approach.