They have a very good queuing system in place at the o2 arena for registered mini cabs. There is a small booth set up near the main exit/entrance at the o2 to order your mini cab, they tell you the price there and then and give you a number, you then go outside and wait for your number to be called. Obviously due to demand, the wait can be a while, I often find it best to go have a drink or two and wait for it to have died down a little and the masses have already gone home.
There is also a black taxi rank near to North Greenwich station, same applies there, leave it a little while after the concert, go have a drink and then go queue for your taxi.
A registered mini cab is a normal looking car, where the driver has had to clear various checks to ensure he can drive the public around, they all carry id to confirm this, please ensure any mini cabs you get in London are authorised mini cabs. There may be unauthorised mini cabs hanging around the o2, which isn't unusual when there are lots of people all trying to get home, they hoping to catch an illegal fare, the o2 security are usually very good at removing them, but please be aware and only get in an authorised mini cab.
A black taxi is the famous black london taxi...just in case you are confused.
