I vaguely rememberering that there are half a million people in the UK with HGV licenses. But only half of them are working as drivers. So it really isn't likely to be a supply problem. It seems to me that the demand is simply not consistent enough to generate the required supply.

Or to put it another way, the market is working as markets do.

If people really want HGV drivers then they will have to offer more, and not just money but better working conditions and more secure employment.