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.