The London transportation system is as efficient as it's expensive. Be sure to get an Oyster card, event if your stay only for a week-end. The journey price is way more if you don't have it, plus you can get fully refunded.
Extra tips :
- Prefer Buses over the tube, it's cheaper and you get to see the city.
- If you have a smartphone, make sure to download the london tube app, to get the offline map of the transportation system.
- On weekends, a lot of the lines are on maintenance, so watch out when you are planing to go somewhere.