Oxford is amazing and I'd admit it might have the edge on Durham for English purely due to the Oxbridge effect. Buuut Durham is hot on its heels and supposedly has the better course.
I didn't turn down my History and English offer last year, I desperately wanted to go but my school messed up one of my A-levels hence didn't make the grades.
This year I was amazingly lucky they even looked at me, let alone got an offer. With Castle I think its a case of the early bird thing, if you get accepted by the department early enough you stand a chance.