Quite cheap as well, but you should be prepared to spend quite a lot of time sitting in a not very confortable position. Visit Eurolines
If you are arriving at Stansted or Luton airports, Terravision offers a cheap coach transfer to central London
Stansted Airport - London --------- Luton Airport – London Victoria
Car:
mmmmmmm well if you insist. Drive on the left, be prepared to be lost (get a AtoZ guide, the best few pounds you can invest to stay roughly on track) and be also prepared to spend a lot of money on parking charges, not mentioning the congestion charge which is now of £8 per day to drive in central London. SeaFrance should give you some info about ferry crossings.
Train:
We recommend the train. Eurostar will take you from Paris to London in less than 3 hours, and you'll experience being in a train under the channel. If you see a whale let us know...
Seriously, we love it. Style, confort, straight from city centre to city centre, what else can you ask for ? Tickets this way : Eurostar
Flying:
If you are coming from a far away land, there might not be a choice. Flying is also good, if sometimes a bit on the expensive side. Heathrow terminal 5 is nearly finished, and their are quite a few other airports around London.
FlyBMI is a good company with very interesting prices. Have a look at their website.
Bicycle:
Yes indeed. No, we are not jocking. It is feasible. If you're coming from not too far, and even from continental europe. For foot passenger, using the Ferry is one of the cheapest options. More informations on SeaFrance website