Following a freedom of information request by Derek Flewin, Ofcom has made available a list of (unassigned) amateur radio call signs

Requests like this have been made in the past and Ofcom provided a list of callsigns that are available to anybody who has just passed a Foundation, Intermediate or Full exam.

“We no longer hold a list of available amateur radio callsigns, as we now use a system that randomly allocates callsigns upon request”

— Ofcom Request

For someone who'd just passed their Advanced exam, these lists have shown them that Ofcom would issue G0, G1, G6 and even G3 call signs if they asked for it via the online system.

Full Freedom of Information Request - https://www.whatdotheyknow.com/request/unallocated_uk_amateur_radio_cal_4#incoming-1071737