Would the CFI be paid for the work? If the answer is yes then they cant do it as they CAA does not validate FAA CPL on UK aircraft.
I have been thinking for awhile about offfering BFR etc in the UK as there are a lot of FAA pilots around but as I understand it to get paid it would have to be in an N-reg aircraft and meet loads of other requirements. Places like AFTS at Norwich do run FAA courses in N-Reg Aircraft in the UK.
Doing it unpaid is an answer I don't have and look forward to others views!