Thanks everyone for your thoughts. I was thinking I'd go to Oxford, but wanted to see if anyone thought I was doing something really stupid - seems not! Juneau, I pretty much agree with what you've said.
Silence, a good and fair point about the possibility of not doing so well at Oxford, but I'm confident I can get at least a 2.1 (with some hard work!). Also, you mentioned you spoke to a MC partner who did the same degree; did the partner say anything else about it that you've not already mentioned?
As for the degree, provided you do the core modules, it will be a qualifying law degree (it's specifically listed on the law society website) and I definitely won't have to do the GDL.
Just one other thing, I've asked the advice of a couple of my friends who've done law degrees, not at Oxbridge, and they initially assumed that I wanted to do the degree so as to avoid doing the GDL because the GDL's so intense. By all accounts, they're both very intensive, and I never thought about that aspect when I applied. But, do you think people could view my intentions as such, and that not doing the GDL could weaken my CV?
Thanks again!