I'd highly recommend Mel Capleton of Open Air Fit.
He works outdoors (as the name implies!) on Wandsworth Common and is incredibly effective and motivating. Both his personal training sessions and group football fitness classes are brilliant and I speak from personal and happy experience!
If you're a regular on Wandsworth Common, you'll no doubt have seen him and his team. They wear blue t-shirts and seem to know most passers-by!
http://www.openairfit.com/