Bloom Labs do bookings for Brits, and you can book online which is what i have done.
However, one thing to watch out for, and not sure if it is a problem at my end, the times that you are given are UK times, not NYC times. So i booked for 1430 on the website, not realising until they sent the confirmation through that it was actually booked for 0930.
Went back in and booked 1815, and came back as 1315 confirmation.
Also - i have asked before but testing anytime on Wednesday for a Friday departure is fine still?