Well so far in this country I've seen 1 C6 (Bahamas)-registered G-IV, 2 VP (Cayman)-registered Citations & 1 N-registered Learjet. They were all operated by their owner for private trips. N-registration is mostly used here for tax avoidance purposes, but to my knowledge there's no commercial operator flying N-registered aircrafts in the UK (or France, or Switzerland, or Denmark,....).
U might have a bit of luck if U are FAA-rated on a specific type though, as some european contract agencies regularly look for C550- or HS125-rated FAA guys for quarter-sharing companies like Netjets, but they also usually require a total time exceeding 2,000.
A few N-reg ones I know of :
1 G-V operated by BMW FlugDienst (Germany)
1 DC-3 operated by Air Dakota (Belgium)
4 B727 operated by EAT on behalf of DHL (Belgium)
1 King Air 200 operated by Sky Service (Belgium)
1 MD11 operated by Air Lingus (Ireland).
MF