Steel is cheap, and extra displacement the same.
If you aren't cheating, you aren't trying to win, as goes the saying in the Sports World.
If the UK gets caught bending the rules then the US, Japan and others will all start cheating too (yes, they probably will anyway but they definitely will if the UK cheats), then the UK has to react to the new foreign designs with their own designs, then the foreigners will react to them and you're back in the same arms race the treaties were supposed to stop. The UK can't afford to be in an arms race, especially not with the US, and they can't afford to fall behind foreign navies.I think it's more a case that as the only new Battleships built from scratch following the Washington Treaty their construction was under intense international scrutiny. Any cheating would have been caught.
I agree that steel's cheap and I agree there's a risk they would have been caught if they'd tried cheating. That's why they literally can't afford to even think about trying.