I think a lot comes down to how you define 'value' - the OP appears to use it in a monetary term; will I get the same ROI if I spend x at Oxbridge vs spending x at a newer uni.
I went to one of the more successful ex-poly's and, whilst I loved my time at uni, my fond memories aren't generally of the university or my studies. It felt rather functional - I learnt stuff, it interested me, I got a degree. It didn't inspire me though.
I went to a friends wedding last year at one of the Oxford colleges, where they had studied previously. Walking around the college for a few hours inspired me more than three years at my uni - I think the experience of the established unis give you an experience you could cherish for life.
In hindsight, knowing how much I now wish I could have experienced being in that inspirational type of environment, surrounded by hugely intelligent people, would have motivated me far more at school than perceived value in a monetary term.
As for my experience, I got 4 c's at a-level, studied a life science at an ex-poly and left with a high 2:1 and took a job in a generic grad scheme job in an unrelated sector. Getting a 2:1 made up for my previous tardiness at college and gave me access to the grad scheme jobs (milk round), most of which are not degree specific. It's worked out fine, I'm not yet 30, very happily living in the south west, earning over twice the national average and doing a job that is reasonably tolerable - but not inspirational.
So, if earning potential is your measure of value then I dont think it matters too much where you go to uni (although the mega money jobs might be more selective). If you want to spark your passion in a subject, be inspired and value more than just earning potential, the more elite unis might be worth holding out for.
I certainly regret not knuckling down at college now for regret of missing out on that aspect.
