I'm 50/50 as to whether this is someone taking the mickey from Mumsnet or something.
Here are a few basics:
Fixies as a rule are overpriced.
Bicycles are very simple things – things like brake failure can be avoided through some basic care and servicing. Things like changing the cables once in while.
Second hand "racer" type bikes are often very cheap and a good way to get around, as they're not horribly likely to get nicked.
Ratty old mountain bikes are similar – nobody's going to nick a mountain bike with rigid forks, V-brakes and commuting tyres.
Bikes are very simple (I know I'm repeating myself). Buy a book on bicycle maintenance and you can make sure your bike is safe and works well for very little outlay. There is only one proviso – buy something really cheap, and no amount of work will make it into a safe bike, as the parts are such poor quality that they're not fit for the job.
Edinburgh Bike Co-op (they have branches outside of Edinburgh too) and Decathlon both sell decent bikes for what you want. The Decathlon B'Twin Riverside 3 looks ideal, costs £199, has a rack, dynamo lights, lots of gears and proper brakes. I believe Decathlon also have good warranties and servicing.
Ridgeback also make some decent bikes in this price range.
You won't be able to pick up anything new with drop bars that is any good on the cheap. Decathlon's cheapest road bike is £270. Racing bike brake levers and shifters are just too expensive.