Or are Brit plumbers simply overvalued and Eastern Europeans represent the true value of plumbing? I’m not sure if I should put a /s here or if I actually believe this.
That Brit plumbers are more expensive does not mean they are overvalued. It means they are not cost effective, and that means they'll get less jobs. You can only be overvalued if anyone is actually paying you for your job. The reason that they're expensive is that cost of living is too high.
So the real question to ask is, do British plumbers expect a too high quality of life?
Until the foothold is gained, and then the immigrant's prices start to go backup when they establish themselves better in their new home. All of this is still a market force, and is how it works in general. On another note, I think you need to see a list of what people think they are gaining for a good comparison to this list of what you are potentially losing.
So the real question to ask is, do British plumbers expect a too high quality of life?