N Reg in the UK
FAR Section 47.3
Requires that an aircraft is eligible for US registration only if it is owned by:
1.) A U.S. Citizen
2.) A permanent resident in the U.S (green card)
3.) A non citizen U.S corporation, if the aircraft is based and used primarily in the U.S. (this means 60% or more of it's flight time is in the U.S
There are other relevant sections dealing with trusts and trustee, FAR 47.11
Regarding the 60% rule, the FAA will track your aircraft every 6 months and monitor the use (Form 8050-117)
For my particular case this is ample enough time.
Regards