Handing keys back to the bank simply is impossible at French law. This would not be a good idea anyway. You cannot personally go Bankrupt in France unless you are a French citizen, or a foreigner whose main place of residence is France. If one decides not to continue paying the mortgage and return the keys to the lender, the latter will initiate foreclosure proceedings and the property will eventually be sold at auction, which could result in the owner remaining liable for any shortfall, including mortgage penalties and auction charges that may not be covered by the auction proceeds. I am unaware whether it is, or not, lenders’ policy to enforce payment of loans/mortgages (the shortfall) against borrowers situated overseas. A Judgment would have to be obtained in France first and the process for enforcing such Judgment in the UK or Ireland, is called ‘Exequatur’. The procedure for this between EU Countries is simplified nowadays. I have little doubt that foreign Courts would accept such requests from a French entity. A UK Judge for example, could not re-assess the case, unless the French Court Order contravenes essential legal principles of UK law, which is extremely rare in financial & tax matters.
Pursuant to the French Consumer Protection Code, borrowers can apply for a ‘repayment holiday’ (deferred payment) of up to 24 months if they are suffering financial hardship. Where the mortgage in arrears is less than 3,800 Euro, a simple Application, rather than a Summons, can be filed with the local Court. This, if applicable, must be done prior to lenders formally issuing a notice of foreclosure, i.e. that the borrower is defaulting on his/her loan repayment.