It’s unlikely that whoever this is is actually not based in the UK.
This is harassment which is an offence under the Protection from Harassment Act. It should be reported to the police and I would suggest that you printed off the relevant pages and took them with you.
It is likely that this would eventually be followed by requests for money which is then blackmail, also a criminal offence.
You should also report the user to Facebook who will block the account.
Ultimately, the police are going to need a physical address if they are going to prosecute the person and it’s unlikely that they would get that.
I can tell you however that Facebook do take complaints of this nature very seriously and I would be very surprised if the account had not already been suspended.
I’m not certain whether that answers the question for you or not but I am happy to answer any specific points arising from this.
Please do not forget to use the rating system so that I get credit from my time here today.
The thread does not close, it remains open if you want to follow up anything or need clarification of any points. Kind regards