If you are very quick, then the US CBP are holding a UK Enrollment 'Event' for Global Entry at the US Embassy in London between 7th March 2018 and the 3rd May 2018.
https://uk.usembassy.gov/global-entr...-event-London/
Good News! U.S. Customs and Border Protection are hosting a Global Entry enrollment event at the U.S. Embassy in London starting March 07 until May 03, 2018.
The enrollment center will be open Monday through Friday from 8 a.m. to 4:30 p.m. CBP officers will only interview Global Entry applicants with appointments.
The U.S. Embassy is located at 33 Nine Elms Lane, London SW11 7US.
Appointments will not take place during the UK holidays Good Friday (March 30) and Easter Monday (April 2).
You have to first apply via gov.uk here:
www.gov.uk/global-entry-usa
and pay £42, then $100 to the US CBP once your UK clearance comes back (usually a couple of days) then it took about 10 days before being able to arrange the interview in London (always providing there are still slots left). For example, I got my US 'Pending' clearance back on the 6th of February and was able to book a slot at the US Embassy for the 13th of March. There were still a lot of slots available after the 13th of March until the 3rd May albeit some days were fully booked.
BM