The King's Head, Woodstock
There are 3 vegan choices here - the vegetable curry, the mushroom stroganof, and the classic baked potato with baked beans (but remember you'll need to ask for no butter)
I recommend the curry!
The only problem is deserts, nothing is suitable, there isn't even a fruit salad. But the main meals are so big that you probably won't want a pudding anyway.
An added bonus is that the staff are really friendly and the service is excellent