Iwas a courier for 5 years worked for Addison Lee on an open call circuit, money was good once you'd learnt your way around Soho & the city.
Best to do lots of minimums (under 3 miles) together upto 12 jobs per hour,thats where i earned the best money.
Once you've been there a while,if your any good you generally get offered a "Guaranteed minimum" per week.