A few more from the UK:

Rebecca Clarke (don't have dates to hand - on tour) - Violist, spent
much of her life in the US - fine chamber music, best known piece Viola
Sonata of 1917

Elisabeth Lutyens (1906-1976 - v prolific - exquisite craftsmanship -
pioneering British serialist.  Active in all forms including many horror
movie scores.  Late chamber music particularly fine.

Elizabeth Maconchy (dates similar to EL above)- justly highly regarded
for her string quartets, strongly rythmic and dissonant writing.

Paul Silverthorne