I have had Global Entry for the last 3 years and it has really transformed my travels to the US.
Since Global Entry was permanently announced for UK citizens, the final (minor) annoyance was that you still needed to complete the application centre at one of the enrolment centres, mostly based in the US.
Well for 60 days from 26th September 2016 (which by my calculation comes to 25th November 2016), there will be a temporary enrolment centre at the US Embassy in London from Mondays to Wednesdays from 8am to 4.30pm. You will need to apply through the usual online methods, but when selecting your interview appointment, you should be able to find London during the above dates.
I can wholeheartedly recommend getting Global Entry. At $100 for paid to the US, plus the £42 processing fee paid to the UK, it gives 5 years of (relatively) hassle-free entry to the US, and it also comes with pre-check for speedier security checks when travelling within or exiting the US.