Jan, I haven't RTFT but our income and circumstances are very similar and my DS started at Birmingham University this year, so I will explain how it worked for us. You are right to be concerned, it was a huge and horrible eye-opener for us.
On an income of about 50k we, or rather he, gets a maintenance loan of £5091 a year. He was lucky in that he got his first choice accommodation, which is a self-catered complex walking distance from campus (Ashcroft in Pritchatts Park). This is the cheapest available university accommodation at Birmingham and is £3889 for the whole year except the summer holidays and he doesn't have to move out at Christmas or Easter. I think it equates to about £92 a week, but on the plus side, he has no regular transport costs. From the loan this leaves a surplus of just £100 a month to live on, which is not enough, so we top this up with £300 a month from our own money. This is working out quite well and if anything is a bit too much. We find it manageable, just, but with DS2 just 2 years younger I have no idea how we will afford it then.
The other shock we got was when on results day 18 August, amid the celebrations, when the accommodation offer email arrived, we found that we had to pay £550 upfront to secure it even though he got no loan money until 26 September. I guess this is in the small print somewhere but I hadn't realised it. Then the rest of the £3889 is split into 3 termly instalments, a few weeks after the loan money, which also comes 3 times a year So the loan money came on 26 Sept and we had to pay £1113 on 1 November by direct debit.
I agree you can probably get non-university accommodation cheaper, BUT it does detract from the whole fresher experience, plus (so far) uni accommodation has been completely hassle-free, no bills, warm, hot water, launderette, and no transport costs. And no landlord hassle.
I was so relieved when he got his first choice accommodation, because there is no guarantee at all you'll get what you apply for and some of his friends were allocated very luxurious en-suite rooms at an extortionate cost. So you have to keep that in mind too.
Some other snippets of advice: avoid London completely. Costs are much higher. If poss choose somewhere not too far from home as train fares even with a railcard are ridiculous, and petrol costs are high too (I'm talking here about taking/fetching at the start and end of term, coming home for the weekend etc). The realistic prospects or working during term time will depend on the course. My DS is doing a subject where they have weekend assignments and they've just been away on a trip for a week, so any employer would have to be very flexible.
Anyway, hope that helps. Good luck.