If its just cycling to work you are looking at then a Hybrid is the steed of choice.
You will only need a double chainset (that a big and a little ring where the pedals are :-)) any thing else is just extra weight that's not needed.
Make sure the frame is as light as possible
Geta good set of lights for front and rear get a saddle pannier to carry a tube or two and the needed multi tools.
Please don't buy any bike at the 100 quid level because they are normally just crap heavy things that are sure to go tits up at some time. £350 notes will get you a good bike and figure on £150 for clothes and the rest.
Now here the important part go into the bike shop you plan to buy from and ask one of the staff to help out with your position on the bike it will save you from any pain.
Also to avoid any major maintenance bills just buy the Haynes bike maint book and give the bike a weekly checkout e.g. Check tyre pressures and so on
Cycling is a major bug I do about 5000 miles a year mainly on the road, dropped about 2 stone and my Bp is down.
I do a bit of work for British Cycling so if you need any advice just Pm and I will be happy to help.