Humanities tutorials at Oxford are mostly 1:2 (the ‘tute partner’ relationship is an important one!); 1:1 is less common but not rare (I was pretty jealous of the 1:1 tutorials with Roy Foster…); 1:3 uncommon (maybe more for 1st year English). Essays are not typically graded (even indicatively), but students complain when they don’t get marked ones back (for the feedback but also for exam review). Discussing / defending the essay is most of the tutorial. You aren’t graded at all on the tutorial itself, but obviously there is nowhere to hide (except behind your tute partner if you have one- hence their value- you can bail each other out!). You never ever ever miss a tutorial.
One of my grad student collegekids is TAing (physics) this year. Their discussion groups are as @VickiSoCal describes- optional and ungraded, though not as big.
StA’s seems to be neither fish nor fowl- much more American than a typical UK uni, but with aspects that are definitely more UK than US. It’s just it’s own self I guess!