I recall seeing a TAT VFW-614 coming out of Heathrow one afternoon and a Luftwaffe example on a remote stand at Heathrow; never saw any others in the UK.
Just trying to think of the most unusual type I ever saw at Luton on my infrequent visits (usually to/from the PFA at Cranfield) but nothing springs to mind - the interesting stuff was always the visiting biz-jets!