As long as you fly within the UK territorial boundaries and the aircraft is registered to a country bounded by icao regulations. With a uk licence you can fly the machine.
As a Uk national holding a licence issued outside of the uk again yes you can fly within the uk, or within the country of licence issue as long as the machine is also registered in that country.
Perm any two from three. ie National of the country you intend flying within. Aircraft registered in the country of operation. Holding a licence to fly issued by the country of operation.
I hope this helps.