Presumably you can close your flight plan by telephone with your (UK) departure airfield if you don't know the local number?
Er, no, definitely not. There is no concept in the UK of closing a FPL. If a UK ATC unit is asked to do this they invariably do nothing with the information, as there is nothing to do.
I don't think a UK ATC unit will realise the implications if you call from abroad to close a FPL. You really do need to do this from within the country you are in (or adjacent, like Flying Boat, but not the UK!
) They know the procedures and what to do with the information.
VA