What UK aircraft looks like an Islander but with twin fins?
It sounds an awful lot like a C-145, or maybe the 'civilian' version (it's primarily used by militaries), the  PZL Skytruck.

Source

It's used by the U.S. Air Force, so seeing it in Britain would not be surprising - all 3 of the U.S. Air Force's RAF joint use bases are within 100 air miles.


It's twin-engine, with a tailplane and twin fins
Its tail boom does narrow somewhat and distinguish itself, as seen here
It does have somewhat of a chunky body
The wings are fairly thin (front-to-back)