The statement that you will be raising a mortgage to buy the property can only be written into the compromis (the first contract) if it is a French mortgage. Should you intend to remortgage your UK. property for the purchase and anything goes wrong with the plan you will be committed to proceeding with the French purchase. A French mortgage will also leave your UK. property free from debt.
And these are not the only advantages. French interest rates are relatively low and French banks/ brokers can issue you with a pre-approved mortgage statement which will clarify your buying budget and can be a good negotiating tool when it comes to discussing the purchase price.