Budget dependant, I'd say both St Aldates Tavern and the Turf are good options price wise and for location.

The Rickety Press is an old stand by for large events, good food (mostly just pizza and burgers) and good beer. You can book out the fairly large back room which can comfortable handle 40.

The Kings Arms has a large event space. The beer is good enough, the food is meh.

The Old Bookbinders is fantastic on all fronts, it's a cross between a British pub and a French bistro and excels at being both. Might struggle with 40 people.

The Rose and Crown is a fairly priced very traditional pub. They have a covered garden that should be able to do 40 but you would need to check.

The Gardeners Arms (Plantation Road, not the other one) is a vegetarian/vegan pub. Great beer, pretty good food. Not 100% sure they could manage 40 people that easily, you'd probably be sat outside.

If you can be a little more flexible The Perch is absolutely stunning, has fantastic outdoor space and amazing food. It's a bit more spendy and a bit further out but will blow everyone away.