I need to reach central London (Piccadilly Circus for example) from Stansted airport at 8 AM.
Does anyone knows a cheap and easy way to make that circuit? Stansted -> Central London, Central London -> Stansted.
You can take the train or a coach the main couch companies are nationalexpress, easybus to get into London. It is cheaper if your book your ticket a long time ahead, however you don't know how long it will take for you to get past passport control etc.
Then you need to take the tube or the bus, Transport for London has a good website with a on-line planner.
The standard single train if you do not book is under £25 per person (a return is under £35), then you have the cost of the tube/bus. The train runs 4 times a hour at peak times, see thetrainline for timetables.
(It is also a LOT quicker from Stansted by train then it is from Heathrow by tube and Stansted being a smaller airport tends to have fewer delays.)
Easybus is one good option.
If you book a long time (months) ahead, then one way ticket from Stansted to Baker street will be £2 - the prices will go up with time though