I was originally gonna post on here just to do some casual Cambridge > Oxford banter, but then again boat race and all that so... better not. However reading the posts thought I'd stick my oar in.
There are two misconceptions about Oxbridge: the first one is that they are the be-all-and-end-all of good education and career prospects. Clearly they're not. Then there's the other side: that Oxbridge are somehow 'declining'. Oxbridge are here to stay, no other UK university in our lifetimes will be worth as much as Oxford or Cambridge on a CV generally speaking (and that includes Imperial/Warwick for maths). They are amazing for many good reasons, from straight ol' history to the huge amount of money the colleges sit on and have to play around with as well as the universities themselves.
Oxbridge has a work mentality which is unique. No other UK university has the short terms, the short deadlines and in turn the incredible facilities that are in place to support those hard-working individuals. 114 libraries for Cambridge alone including a copy-library and some of the best specialist libraries in the world, I'm sure the figure for Oxford is similarly spectacular.
You go to Oxbridge, you go to university with many normal smart people like you do at other universities, you also go to university with some of the people who will change tomorrow's world, as you would at any other good university. But there are more of the latter at Oxbridge because that incredible work-hard-play-hard mentality is enstilled in such a way that it will for a long time yet be worth an awful lot on your CV.
In terms of the OP's point though, there are many amazing universities in the UK. Undergrad is just the start though, you can always come back to Oxbridge later.