The B22d bayonet cap (BC) is the standard light bulb cap used in the UK. It is mainly found in ceiling fittings, wall lights and floor lamps, and its versatility means that there are many LED light bulbs available with this type of cap. The B15d small bayonet cap (SBC) is a variant used in smaller lamps and fittings.
It is a 'push-and-twist' type of cap, with two small horizontal spikes protruding from its base. It is easy to fit: simply align the spikes on the cap to the slots in the fitting, gently push the bulb and turn it clockwise until it locks into place.