JAR-OPS 1.940 states;
(b) Minimum flight crew for operations under
IFR or at night. For operations under IFR or at night,
an operator shall ensure that:
(1) For all turbo-propeller aeroplanes
with a maximum approved passenger seating
configuration of more than 9 and for all turbojet
aeroplanes, the minimum flight crew is 2 pilots;
or
(2) Aeroplanes other than those covered
by sub-paragraph (b)(1) above are operated by a
single pilot provided that the requirements of
Appendix 2 to JAR-OPS 1.940 are satisfied. If the
requirements of Appendix 2 are not satisfied, the
minimum flight crew is 2 pilots.
[Ch. 1, 01.03.98; Amdt. 3, 01.12.01; Amdt. 7, 01.09.04]
However this only applies to JAR public transport operations (Part 135/129/121). The JAA type raring requirements can be found at
http://www.jaa.nl/licensing/classtyperatings.html
Your best bet for information on US registered aircraft in UK airspace is likely to be the NBAA who are usually very helpful.
MM