its kina not like a moto selection !
you want a basic bike that you won't worry about laving overnight on the street.
so forget about brand names and models and whatnot
second hand is the way forward, cheap as chips and face it, its such basic tech you don't need to worry about 'hidden' problems. Its all so easily sorted out.
riding round london is more like a mountain bike course than the tour de France so get a mtn bike and put slick ish tyres on, 10 pound in decathlon rotherhithe. They hve wider stronger rims than a racer and the tyres have bigger air pockets, so stronger and better for the potholes and kerbs, if you pump em up hard the rolling resistance is not much more than a racer anyway.
don't get disc braks. cable discs are shit compared to v brakes, and the dics warp which is a pain, the ydraulic ones are great but, try and bleed those feckers with their 'borrower'/gnome master cylinder...
come to druid st se1, I'll sort you out mate