Welcome to the forums. I hope you find from your doctor that you can commute without risk.

I don't live in the UK, but if you just HAVE to have a brand new bike, pick up a copy of Cycling Plus magazine. I'm a subscriber and they always have lots of adds in the back for Wiggle, Pearson, Evans, etc., and some of those bikes are under 500GBP. There's a classified section too.

While I agree with the others that you can start with your street clothes, do take into account that once you've been doing it a while, you'll start to want cycling specific gear.

Another thing to consider as you budget...do you need lights? Do you have some kind of sporty rain gear? A backpack? etc. I know these seem like simple questions, but not everyone thinks of these things when they first start and are determined to stick to a budget.

Good luck and happy commuting!