The current problem is U.K.- wide. It's nothing to do with CLDA/Stobart or Loganair. It goes back in fact to the end of WW2, and was 'enhanced' by the 1970s oil-crisis. In the late 1980s, NATS introduced irrelevant 'SHL aptitude tests' for those applying to be ATCOs, and reduced the entry requirements from 2 'A' levels to 5 GCSE 'O' Levels. The pass rate on each course dropped from 95% to about 20%. In spite of a major recruitment drive from 1978 onwards, the 'recruitment bulge' is now a 'retirement bulge'.
The present shortage is a 'global problem'.
Many of those appointed to HR/training positions, post-PFI, had no ATC or aviation experience, and couldn't get their heads around how long it takes to qualify to do the job. Heathrow's best controller cannot walk into Carlisle tower and start work on day 1. Also, new training regulations introduced by the European Aviation Safety Authority are an issue. Way back, a controller's licence was valid for the lifetime of the holder. I believe that is no longer the case?