Just get a nice pair jeans, dark blue are a nice colour. You don't have to spend loads of money on them, just as long as they look okay, smart and aren't the ripped, patched, just got out of bed look.
Some brown shoes would go nice too. Sometimes (unless they have a dress policy) will allow trainers, by trainers I mean the casual smart look not running ones or Nike.
For the top, a nice fitting shirt with stripes would be perfect, usually what most people wear, doesn't have to be stripes of cause.. T-shirts usuauly work unless there is a policy agaist them.
Black trousers also would work but I'd wear probably black shoes and deffo a shirt with them (probably white).
Some good stores are Burton, Next, M&S, eBay (+usually new, +branded, +a lot cheaper than the store, -only photos, -can't try before you buy), check out the stores around your city centre too, Primark, George, Matalan aren't too bad either. I would probably avoid Primark for shirts because you'll notice if you live near a Primark that a lot of other people might also wear them.