What are the rules about take a bike on the London Tube? Can I take a bike on the Tube (underground) in London, if so how much does it cost?
Transport for London page on Bikes on Public Transport

On the tube, folding bikes are good anywhere at any time.  Non-folders are restricted to only certain lines - in most cases these are lines that have no deep below ground tracks - and certain times. 

To a certain extent it is going to be up to the station/train staff.  I've travelled on the DLR with my bike in the past, before I realised they have a total ban (I was dismissed from the train, which sucked because I'd had a few to drink and was trying to avoid cycling home).  If in any doubt, or trying to wing it, its always worth asking before you enter the station.