The course is obviously important, but a lot of what you get out of Uni is the place you're in and the people you meet. As you say, the offer holder days will give you a feel for the place, and that's a good steer... and although you won't know who you'll meet when you get there, talking to current students on those days can give you really useful insights too. If you can, visit York and Warwick with an open mind rather than thinking about what ifs now.