What is the least expensive way to become a commercial pilot in the UK?
There's a finite minimum set of requirements you must meet to get that ATPL and type ratings, and there's a finite minimum cost associated with that.
Nothing's going to change that. The only thing you can possibly, maybe, change is who is going to pay that money.

So don't fail any of the tests. The ONLY way to reduce the cost is to complete the training in as short a time as possible. Which means minimising flight hours, simulator hours, and retaking of courses. All of which boils down to passing every test first try.

And it will still take tens of thousands of pounds.

Going the Air Force route is also takes tens of thousands of pounds, except the air force (and thus other people, taxpayers) pick up part of the bill.
Same with every other scenario in which you're not paying everything yourself. Someone, somewhere, is paying for it.
Used to be (and maybe still is, but it's unlikely given the glut of unemployed pilots) some airlines would take on people and train them from scratch. But that's now rare. Most at the very least require you to have your PPL already, and many won't hire you unless you are qualified on at least some aircraft in their fleet.