I am from the US here, so better versed on the US brands than the UK ones. A hybrid might be nice if you are not keen on going fast, and the slightly more upright position would make you more visible in traffic. I would definitely not get a fixed gear (or 'fixie') if you are not yet comfortable riding in traffic as they take a lot of getting used to for cornering and stopping differences.
Try, if you can, looking at Craig's-list just outside the city as in the city prices are more likely higher. E-bay is another decent source but I would look for local sellers with a pick-up option so that you can see and try the bike before shelling out the cash. A lot of people buy a bike with the intention to ride it and keep it in the garage for years, only to sell it later because it's never ridden and just taking up space. Do make sure though that anything used is not stolen (I made sure mine had the original reciept). I have a Trek hybrid from CList that I love, but looked at a Giant while I was shopping around and it was nice as well. Also invest in a good lock, if you are planning on biking to lunch or to the shop you want to make sure it's not an easy target.