I think London cabs are really cool, they are designed specifically to be cabs and work really well. There is so much room in the back you can seat 5 and all their luggage and then have a nice game of volleyball.
But at night all that empty space can be a bit overwhelming since it's pitch black. Look around when you're seated and you should see a small control panel that has a button to talk to the driver (who unfortunately is encased in plexi) and a light switch. Turn on the lights when exiting to make sure you have all your various and sundries, I nearly left some stuff behind.
And even though cabbies are supposed to have The Knowledge, also carry the printed address of where you're headed. Our cabbie at Heathrow couldn't wrap his head around a Hilton that I said was across from St. Pauls, so I showed him the address and he put it in his GPS and off we went (traffic was quite terrible but never mind that). When we finally got there he looked it up and down and called it by a different name. Come to think, this also happened to me in Vienna with an oddball hotel.