Is there a website for comparing UK dress sizes ( in terms of measurements) for brands?

[Britpopbaby]

Or is it a case of doing the leg work myself?

I don't think there is - it would be helpful if there was!

Mind you, the size disparity within brands can be huge. Yesterday I wore size 20 trousers and t shirt from Marks and Spencers, today my M&S trousers are a 24 and my M&S t shirt is a size 18. It all fits me well.

Sainsbury's is just as bad. I am usually a size 20 in there but recently had to send back size 18 trousers, eventually ending up in a size 14.

It's crazy how sizes differ so wildly within a brand.

No. I think what makes it worse/impossible is that with some brands, there's not even any consistency amongst sizes.

Yes, https://sizes.darkgreener.com has been around for years but I cannot comment as to its up-to-dateness and accuracy.

There's no reliable consistency within brands anyway.