How British reserves differ from the US National Guard

Is the Territorial Army Britain's equivalent of the US Army Reserve rather than the National Guard?

Broadly yes; Britain lacks a direct National Guard equivalent, and its formerly separate reserve organizations were later consolidated.

Britain has no direct modern equivalent to the US National Guard, with the old county regiments offered as the nearest historical comparison. The Territorial Army and Army Reserve were eventually combined, just as the Royal Navy Reserve absorbed the Royal Naval Volunteer Reserve. Before consolidation, the naval bodies had distinct recruiting missions: one drew heavily on former naval personnel and merchant sailors, while the other sought civilians, yachtsmen, and people with useful outside skills. Administrative streamlining can make a reserve simpler while weakening its ability to cultivate separate sources of wartime expertise.