Read the Driving Instructors' Assocation article regarding driving test waiting lists: https://www.driving.org/driving-test-wait-times-have-increased-at-two-fifths-of-test-centres-this-year/
Local Driving Instructors are working incredibly hard behind the scenes trying to organise tests for their students. Candidates often cannot find a test time/date at a time which is suitable. It could be that the candidate is ready for test but having to wait months, or vice versa, not quite ready and with a test too soon.
Instructors are working among themselves to swap test appointments which is very time consuming, but ultimately to the benefit of everyone concerned. Hopefully the DVSA will begin to reduce the waiting times for tests soon.