"It benefits British builders by getting better workers. And unless gardeners, electricians etc live in plumbless houses, it benefits them as well.
The only people it doesn't benefit through direct competition is other plumbers. So what? If you think plumbers are hard up I certainly don't see it. Bring them in I say. Plumbers get paid well or certainly the ones that come to my house do. Anyone who owns a house knows how much building work in general costs, and specifically how much plumbing costs. So, why should the interests of British plumbers outweigh everyone else's?"
You appear to be living in a bubble.
The plumbers that come to my house are employed, and what you pay is not what they receive. If they are self-employed than work is irregular and unpredictable.
I am not sure what you mean by British builders getting better workers. British builders, on the whole, are individuals trying to get the best wage for their skill. So it doesn't benefit them to get more immigrants in to reduce the value of that. How could it?
And if you are a gardener, electrician, whatever, then the £50 you might spend on plumbing in 10 years hardly compensates for the 30% you've had knocked off your wages by immigrants who are of course not merely plumbers, but many different trades, driving down wages in all.
"And part of overall housing costs are maintenance costs. if those go up, so does the cost of housing. Isn't housing one of the main issues we have to deal with here?"
Nope.
The problem is that land is expensive. Houses aren't expensive. It's the land.
A tiny plot of land in Romford with no house on it?
That will be £225,000 please www.rightmove.co.uk/property-for-sale/property-43595725.html
One of the main reasons that land is expensive is because we have so many immigrants and no more land available for building on. So if you admit 100,000 skilled labourers, that might reduce the cost of building your house by £5000, but it adds £50,000 onto the cost of the plot. Not a great idea is it?