UK Telecommunications regulator Ofcom no longer issues lists of unassigned — or unallocated — Amateur Radio call signs. This practice ended last fall.
Replying to an inquiry, Ofcom’s Julia Snape explained, “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.”
A call sign database dated September 20, 2016, can be downloaded as an Excel spreadsheet.