If you are in a bus lane for a very short time, as the op was, and there is NO traffic in it, i.e. taxis, bikes, buses etc you are not causing an obstruction and they should not fine you imho. If you do cause an obstruction in the bus lane then you should definitely expect the fine. 2 reps from Southwark is ott but given they have so much rental debt to make up it is not surprising