As Ethel states, the only vehicles allowed in bus lanes (during their periods of operation and other, obviously, than buses) are those vehicles which are shown, either in words or pictograms, on the relevant signs.
Unless there is a clear indication, on the sign, that motorcycles are permitted, any bike rider using the lane will be liable to prosecution (or a fixed penalty) under Section 5 of the Road Traffic Regulation Act 1984.
As a motorcyclist myself I can say that some bus lanes we can use officially but they are few. However we don't use them as there is no need. We can filter past stationary and slow moving traffic anyway. We can then smile smugly to ourselves inside our lids and reflect on the sad soles waiting in their slow moving boxes, mwwahahaha! (evil laugh)