Aci Europe the airports body has a policy paper that discusses that for airports in Europe with fewer than 5m pax pa, it is extremely difficult to make money...
https://www.aci-europe.org/policy/position-papers.html
This is not just a UK problem, although certainly APD has an impact. But so does security, privatization of airports and so on, all of which are Europe wide.
Smaller airports need multiple revenue sources, passengers are at best a small piece of the pie
FF