In WW1 they held the line against the Germans after the Regular British Army being small in number had been decimated until more soldiers could be conscripted and trained to be sent to France. In WW2 I think they served in the BEF along with the regular British army and along with the those conscripted were folded into the regular British so there was no distinction but don't quote me on that because I'm not 100% on that.
You could also include the Army regular reserve which are ex soldiers of the British army that are still eligible to serve.
The Royal Auxiliary Air Force has elements in the RAF regiment that could be considered members of a well regulated militia. The Royal Marines too have a volunteer reserve too. Think Royal navy does too.
I think the national guard is the US version of a we regulated militia.