Nah you'll be fine. I'm from a really poor background with immigrant parents coming from a rough area in Lincoln, East Midlands. Depending on your college you may be surrounded by more posh people or less. You can do some research before hand to try and select a more relaxed college where you will fit in more. (It does matter if you don't want to be around posh people).
I'm personally in Cuths and in 2nd year rn and its a pretty chill college where you don't feel out of place as someone who is working class. The stereotype about Durham being only for posh people is really strong but I feel like its exaggerated a bit. Posh people are out and about here but "fitting in" is like not even that big of a concern if you just want to live. However if you want to challenge the rich and join the richer/posher societies like the Durham Union (Debating one), you (one) need to pay like £70 or something crazy to join. This membership fee is probs the highest in Durham but the posher the society, the higher cost of membership is a good rule of thumb.
The cost for some of the college events can be quite expensive. The summer ball for example, tickets are like £70 ish for a dinner ticket which is a big hit to the wallet, and if you want to buy a nice outfit or something *rip funds*. My college has a small fund for the working class to help pay for things like the summer ball. This is quite nice but i'm not entirely sure if all colleges do this.
If your worried about your age going in as a fresher, don't be worried. Nobody will look down on you, I have heard of like 26 year old freshers and nobody really minds.
If your worried about money, try and get a job as soon as possible, go around Durham and hand out CVs and try your luck. The job hunt in Durham is pretty ropes so starting soon is the best. However you are aiming to study English and that is quite time intensive, I would recommend bar work in either the clubs or colleges since the hours are out of the way and not too much which is more ideal for students. However I'm pretty sure the application process for college bars are largely nepotism. If you want the job, make specific friends.
Nepotism
The nepotism in Durham Uni is actually so bad. When it comes to societies, colleges and their committees ran by students. I feel like there is alot of Succession like power grabs its funny. So much drama, so much posh people arguing (Especially Durham Union). If you want to climb the ladder in terms of your college JCR/Hierarchy. Just get to know as many people as possible and be really nice, students are the ones who vote who gets a position.
If you like drinking at bars, the college bars are pretty cheap and you can do some nice bar crawls which wont break the bank.
I don't know why I wrote so much, I'm just procrastinating from my formatives lol.