I just thought I should post a slightly more moderate message and say that I hope nobody took offence at might rather negative tone towards London.
I noticed that somebody (the message about the US B+ compared to Oxford A) said that not everybody wants to be an academic and that an Oxford (Oxbridge) education is not necessarily the best preparation for life as an accountant or solicitor. Interesting point! Actually, I rather suspect that for those wanting to go into what I think of as boring jobs that make lots of money, an Oxford or Cambridge degree is possibly more useful than it is in academia. In academic circles people will go to the trouble of finding out how academically acute somebody really is and won't judge them on the basis of where their first degree was from. I knew people doing masters/doctoral degrees at Oxford who had been to UCL, LSE, Warwick, Newcastle, Duke, Yale, Durham. The LSE person was insistent that LSE was as good as, and probably better than, Oxford. The UCL person was sure that Oxford was immeasurably better than UCL. The person who had degrees from both Yale and Duke thought that the important thing was to get a degree from a British university: he claimed that in the US degrees from even the best US universities are regarded as inferior to British qualifications. That seems rather a strange idea to me, especially as we in Britain are forever comparing ourselves with America and lamenting that our universities aren't as good as Harvard.
Well, the point is, that if one wants to go into accountancy or law, I've been told, by Oxbridge and non-Oxbridge graduates, that there is a huge bias in favour of Oxbridge graduates. In academia, I don't think anybody is going to look down on a brilliant scholar because his or her first degree was from somewhere other than Oxford or Cambridge. Indeed, some of the top scholars at Oxford have got their first degrees elsewhere. Peter Atkins went to Leicester, Keith Ward went to Cardiff, and GEM de Ste Croix went to UCL (and interestingly he didn't start his degree until he was 40, having left school at 15 and become a solicitor).
I'm sure that in every respect going to university in London can be a good experience and won't be an impediment for the rest of your life. Personally, I would recommend Oxford (or Cambridge - though I don't really know it) over London, but many people go to Oxford and make nothing of it, so better to go to London and work hard and potentially end up more successful than lazy people who went to Oxford just to have a good time.
Oh, and about the Pimms and punting, I've been punting twice - once on a visit to Oxford before I had even applied, and once after finals. I seem to think that I had Pimms at some time. I certainly had Pimms before going to Oxford but I can't remember a specific event at Oxford where I had it. The main drink I had was instant coffee (fairtrade of course) in polystyrene cups, until the JCR decided that for the benefit of the environment members were expected to bring their own cup or mug to the JCR.