I found this on Southgate ARC's news site, but thought it may interest anyone taking, or thinking of taking an exam.
Until September 2016 Ofcom would issue lists of unallocated amateur radio call signs to help people select a new call - they have now stopped doing this
On December 9, 2016 Mike Lewis wrote to Ofcom asking for a list of available call signs, on December 20 he received a reply from Julia Snape which said:
We do not hold a list of call signs that are available. Due to a system change, the assignment of call signs is now done using an algorithm rather than “grabbing” from a list. We have a historic available call sign list which was disclosed in July 2016 (but is now likely to be out of date) and which can be found here:
The link the Ofcom reply quoted in their reply of Dec 20 was a broken link, however a call sign database dated September 20 can be downloaded as an Excel Spreadsheet from which unallocated calls might be deduced