Looks like you can buy it digitally here:

https://www.microsoft.com/en-us/p/tecmo-bowl-throwback/bnh6f3hfg75f?activetab=pivot:

EDIT: oh, but I just realized your in the UK, so it could be a region thing.  You could always go through the hassle of switching regions to buy it: https://www.xboxachievements.com/forum/topic/441739-purchasing-xbox-one-games-from-a-different-region/

I know it's not the best game, but I really want to try it, and I've read that it's backwards compatible.

I found the Xbox marketplace page on mobile, but when I click to buy it says it's not available in my region (I'm in the UK).

Is there any way I can get it, or am I out of luck? I don't really know how region locking works with BC 360 games...

How to get Tecmo Bowl Throwback?