The interesting thing in London is that actually some of the bus lanes you can use, even if the sign doesn't stipulate motorbikes. For example, in Islington they recently implemented a trial that allowed you to use some of their bus lanes, whether signed or not.
I frequently travel through Islington for work, unfortunately, if you actually read the guidance it's more complicated than it even seems from first look:-
Motorcycles In Islington bus lanes
Transport for London are currently conducting a trial to let motorcycles, mopeds, scooters and tricycles - but not those with sidecars - travel in most red route bus lanes in London.
This means that the above vehicles will be allowed to travel along all Red Route bus lanes in Islington without receiving a penalty charge.
However, non-Red Route bus lanes are not included in the trial and therefore the rules in Islington bus lanes that are not on Red Routes will remain unchanged. Non-Red Route bus lanes can be found in streets such as Essex Road, Caledonian Road, Old Street, Junction Road, St Paul's Road and Balls Pond Road.
To avoid getting a ticket please check the signs first to see if it’s okay to use any bus lane before entering it.
Taken from:
http://www.islington.gov.uk/services...e_parking.aspx