BUT the real reason is that they can't find decent work!
I'd say decent work at a decent price. In Poland a Native Speaker is something prestigious, in a way. In UK, they are often just another English speaker with a CELTA. Summer work in places like Paignton earned me a lower hourly rate than in provincial Poland.
the other is lack of a relevant education. A degree in english lit (or some other useless program) and a celta certificate + a few years of teaching grammar isn't going to get you far in today's competitive world (outside of the esl market).
This depends. Another problem is that unless you are somewhere like the south-east, where there are a lot of schools, language schools aren't THAT common in UK. I mean, if I go into a small town in Poland i can usually find one or two schools. In UK it might be impossible to find any. Even those that are large enough to take new staff probably deal mainly with summer work, so the jobs aren't always there. Of course, it doesn't help when whatever experience and quals you have as an EFL teacher are so lowly regarded by 'real' teachers that you then have to do a 'real teacher's' course to get work in the state school sector.
If you people (those complaining about the UK) had degrees/qualifications in in-demand jobs (science, engineering, medicine etc.), you wouldn't have any problems readjusting.
Not necessarily. A few years ago there were very few good engineering jobs in UK and the amount of engineering studes was going down. Some couldn't even get decent practical work as degree students. Likewise, many medical professionals, especially nurses, have left UK to go abroad to get better paid jobs with better conditions.
As for the type of degree you have guaranteeing anything, I did my CELTA with an English engineer in Warsaw. He'd been offered a job in Poland, come over and moved his life here... then the firm went bust, leaving him high and dry, with a flat, girlfriend and other expenses. Not speaking Polish he had to find another way to make money... teaching English.