I assume the house in France is owned by the person who owes the money. The collection agency themselves cannot make them sell the house, any more than they could make someone sell a house in this country.
However, the creditor could go through a legal procedure which could result in the debtor being made bankrupt. If that happened, the house (and any assets of the debtor worldwide) would be for the Official Receiver to dispose of for the benefit of creditors and would very likely be sold.
Alternatively, the creditor could go to Court in this country and get a CCJ. If the debtor did not then comply with the terms of the CCJ the creditor could (through the Court) get enforcement action taken in France. I do not know what type of enforcement French law allows - if it allows for houses to be taken in this situation then it would be at risk.