I wouldn't wear a suit. You can ask the recruiter or HR person what is expected of you, but in games it will typically be casual. Jeans and a polo shirt are almost certainly going to be perfectly fine, unless you have a particularly different cultural norm for this sort of thing in the UK.
As for the test, I generally recommend that people don't really stress so much about "studying" for these kinds of things. Well-written programming tests aren't about probing your knowledge of particular boilerplate algorithms or facts unless you're looking an extremely specialized job, so trying to cram for them is probably not going to be to your benefit. Programming tests tend to involve either basic fundamentals with a language... which if you feel you need to study up on prior to the interview may suggest you're probably not going to do well at the job even if you got it... or trying to mock up real-world scenarios that a programmer might find him or herself in. In both cases, "cramming" can be fairly easily detected during the follow-up talks about the test. Or you might study the wrong thing completely (the domain of computer science and development topics is massive) and then be extremely demoralized when you get the test and it's about nothing you bothered to prepare for, thus hurting your morale and ability to execute. Or worse, you can successfully game the interview and get the job and then do extremely poorly and get fired because you managed to present a false sense of your own abilities.
I would spend the time before the tests trying to relax and not stress about them overly much. I know that's easier to say than to do, but there it is.
