Get out into the countryside and find some real hills. Once you've done this a few times you'll never whinge about anything inside London.
Try:
a) The road from Ewhurst to Shere in the Surrey Hills

Ranmore Common Road from Box Hill & Westhumle Station up to Ranmore Road.
c) The road from Holmbury St Mary to Ranmore Common (White Down). Although the prolonged 17% section was too much for me and my 71" gear. But I'll be back to defeat it when I flip the wheel to 46x18.
d) The B269 all the way from South Croydon to Limpsfield and back. That hurt too!
Further afield but doable by train:-
a) Aston Clinton near Aylesbury, South on B489, R onto B4009 and then left up and over to Chivery. Lather, rinse and repeat.

North out of Pangbourne on the B471, L onto B4256 through Goring and up Streatley Hill, L back to Pangbourne through Ashampstead.
This route:
http://www.bikely.com/maps/bike-path/Cheam-and-Morden-Hilly-50154877 (shamelessly stolen from the Cheam & Morden CTC group) is my new fixed training ride. My God it hurt first time but the difference on a sucsequent 600km ride was astonishing; previously notable hills seemed to disappear.