I'm not sure about Oxford, but I remember I considered applying for universities in London - accommodation was expensive. For example, with universities such as KCL/UCL/LSE, there was no accommodation onsite which meant you had to seek accommodation elsewhere. I was not keen on this as I didn't want to travel around London alone. Accommodation costs were around £200 per week (but this was a studio room).
In terms of eating out, it really does depend on where you go - restaurants are likely to be more expensive than regular take-outs. Fast food shops are cheap (McDonalds, Dixie's, KFC, etc) but you've always got the supermarkets/meal deals etc.
Don't be too put off though - there is always support available from the university, including bursaries/funding, etc. You also get your maintenance loan etc. You'll be fine, just choose wisely.