Getting into town from other airports

Heathrow
Ridable, just go east till you get to central London or you can catch the tube as long as your bike is still boxed (much cheaper than the "Heathrow express" train and goes more central)

Gatwick & Luton
Proper trains go directly to central London for not too silly money, its too far to ride with much luggage from these airports. You can take your bike on these trains unboxed out of rush hour. From Luton airport there is a free bus which takes you to the train station.. bike needs to be boxed, but its not far to ride.