Sorry, don't understand where your 406 comes from.
There are basically three ways to measure COVID deaths
(1) "within 28 days" - quick and dirty but includes people incidentally hit by a bus, and excludes people whose death is caused by COVID but live longer than 28 days.
(2) "On the death certificate" - probably most rigorous but takes a lot longer for the stats to come through
(3) "Excess deaths" - ignoring whether caused by COVID or not looks at how many more deaths overall there are than is normally the case
(2) is higher than (1): 158,664 compared to 136,138 according to that link to the ONS.
(3) is most often used for international comparisons and shows similar trends to the other measures.
There's loads of nuance in all of this, and I don't at all claim to understand all of it. But there doesn't seem to be any evidence whatever that the UK is somehow overcounting COVID deaths either relative to what doctors conclude on death certificate or by other means of international comparison. It's a red herring.
Much more here
Quality and methodology information for deaths registered weekly and monthly in England and Wales data. Includes strengths and limitations, methods used, comparability with other sources, definitions and the effects of the coronavirus (COVID-19) pandemic.
www.ons.gov.uk