There's been a triple whammy going on over the last few years.
1) With newbuild still in the doldrums after more or less shutting down back in 2009/10, there are a lot of blokes who used to be doing site work trying to make a living in the domestic RMI sector
2) The relaxation of immigration controls and the accession of the east european states to the EU, has caused a large number of incoming migrants.
3) The silly media stories about plumbers earning £££££s spawned a raft of training agencies to meet a ridiculous and unjustified demand. They now have their overheads and payrolls to meet, so although the media stories have slackened off, these training agencies pump up the marketing hype to keep the flow of wannabes coming.
All this means that supply is outstripping demand - and if established tradesmen are struggling to find enough work, they aren't going to be in a position to take on apprentices.
For what its worth, all three factors will ease off - in fact I think some of them already are easing. There is an unfulfilled housing demand that is slowly starting to be felt in the new build sector - although its very patchy. Some of the migrants have returned home, as movements in exchange rates and improvements in their domestic economies tip the balance. Finally, I think that more training agencies are going broke than are starting up.
Out of each generation, some people will enter the labour market at a high point in the economic cycle, and some will enter at a low point. I know that this isn't much consolation now, but its just a fact of life.
I entered the job market in the early 80s, and had a series of rubbish jobs which just about paid the rent. I think most people of my age had a similar experience. I had a mate with a degree in astrophysics who was working as a motorbike courier, because it was the only job he could get.