Sod buying shite! Be patient, save up, hope you dont die in the mean time and buy a proper one.
The way I see it is you could spend £200 on a piece of junk and end up wasting your money. It may also put you off cycling. Spending more money means you get a better deal (usually) and therefore more likely to be worth what you spent. You will almost certainly get more pleasure from a better bike. This means you will end up riding it more which will benefit your health too. Think around £600 for something tidy.
The way I see it is you could spend £200 on a piece of junk and end up wasting your money. It may also put you off cycling. Spending more money means you get a better deal (usually) and therefore more likely to be worth what you spent. You will almost certainly get more pleasure from a better bike. This means you will end up riding it more which will benefit your health too. Think around £600 for something tidy.