It has nothing to do with the RAF.
The notation stands for: Lance Bombardier, Royal Artillery
EDIT: Bombardier is a rank used in artillery units in the armies of Commonwealth countries instead of corporal. Lance-bombardier is used instead of lance-corporal.
Bombardier (Bdr) and lance-bombardier (LBdr or L/Bdr) are used by the British Army in the Royal Artillery and Royal Horse Artillery. The same applies to the Royal Australian Artillery, the Royal New Zealand Artillery, the South African Army Artillery and the Armed Forces of Malta. In the Canadian Forces, the Artillery Branch uses the ranks of master bombardier and bombardier instead of master corporal and corporal. In the Australian Army the insignia is identical to that of a corporal, the only distinguishing featuring being the RAA badge worn on the cap by lance bombardiers and bombardiers.
Originally, the Royal Artillery had corporals (but not lance-corporals) and a bombardier was junior to a corporal and wore a single chevron. Unlike a lance-corporal, a bombardier held full non-commissioned rank and not an acting appointment. The rank was equivalent to second corporal in the Royal Engineers and Army Ordnance Corps.
In 1920, corporals were abolished in the Royal Artillery and bombardiers became the equivalent and acquired the normal two chevrons.
The rank of lance bombardier originated as acting bombardier, an appointment similar to lance-corporal which was also indicated by a single chevron. The appointment was renamed lance-bombardier in February 1918 and became a full rank, as did lance-corporal, in 1961.
"Bomb" is widely used as an abbreviated form of address for both full bombardiers and lance-bombardiers. They may also be referred to as a "full screw" (bombardier) or a "lance jack" (lance-bombardier), in common with corporals and lance-corporals. As with other common military abbreviations, such as "sarge", these terms are not used on formal occasions.