Can I take a bike on the Tube (underground) in London, if so how much does it cost?
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, although the above page doesn't specify these very well. In fact it says you can take your non folding bike on the Overground at any time, yet I saw a sign in Golders Green station last night stating the more or less standard ban between 7 and 10 am and 4 and 6 pm.
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.