Before revision:  I estimate that, among people aged 45+, 610 405 excess deaths were registered from 21 March through 27 March, 5689 \pm 420 excess deaths from \pm  28 March through 3 April, and 9374 \pm 470 excess deaths from 4-10 April . The ONS\pm  's respective counts of 531,3432, and 6139 COVID-19-associated deaths are appreciably lower, implying that the ONS's counts underestimate rather than overestimate the true death toll.
After revision:  I estimate that, among people aged 45+, 610 405 excess deaths were registered from 21 March through 27 March, 5689 \pm 420 excess deaths from \pm 28 March through 3 April, and 9374 \pm 470 excess deaths from 4-10 April , and 12,712\pm 's respective counts of 531,3432, and 6139 COVID-19-associated deaths are appreciably lower, implying that the ONS's counts underestimate rather than overestimate the true death toll.