8.5 mile Commute what bike?

[BatterseaV8]

Well my budget isnt huge (£150ish) but I am looking for a place where I can buy (second hand) a bike that I will be able to commute most days from Battersea to Canary Wharf. First time cycling around the city so any comments apprecaited plus ideas of where I can buy my new form of transport gumtree does kick up the odd few here and there.