I don't know the origins of it, but an L is used to denote British Army weapons (apparently it stands for Land Service) and is used for everything from pistols to 165mm guns. As others have stated it can get confusing for the same combination of numbers and letters can refer to different weapons, so it's used in conjunction with a description, e.g. L1A1 Self Loading Rifle. A higher number does not automatically mean that weapon is newer - e.g. the L118 light gun came into service long before the L85 assault rifle or the L105 pistol (I suppose it's the same as the US, with the M4 being newer than the M16).
Wiki has a listing of the more common L numbers
http://en.wikipedia.org/wiki/List_of...apon_L_numbers