http://www.tfl.gov.uk/gettingaround/tran.../1171.aspx

"Wheelchair users have priority over everyone else for use of the wheelchair space. If someone in a wheelchair wishes to board, and the wheelchair space is occupied by standing passengers or buggies, standing passengers will be asked by the driver to make room if possible, buggy owners will be asked to fold them and put them in the luggage space or keep them by their side."