"If they claim only to have had sex in France there is no offence as the age of consent there is 15.
Or is there?"

Actually as far as at least part of the UK goes it would be an offense. If a UK national carries out a sexual act abroad which is illegal in Scotland it is an offense even if not illegal in the country where the act took place. Sexual Offences Scotland Act 2009. It was basically introduced to deal with sex tourists to the likes of Thailand etc but would no doubt also cover the above example.