Mr

Has anyone tried the uk 18v xr battery in a 20v max tool

You shouldn’t have any problems except warranty. 20v max and 18v are basically one in the same. 18v is actual voltage under load and 20v is basically a nominal voltage essentially the equivalent of a 2×4 board and a 1-1/2 in x 3-1/2 in board.

I have a mix and match setup of UK and US sourced Dewalt 18/20 and 54/60 volt tools.  I mostly now buy bare tools on my regular trips to the US, most recently the new 60V in-line blower, not yet available in UK.

As Jronman says, you’re all good to go, bar the warranty, so if you’re not particularly hard on your tools, you should have no major issues.