I don't really understand the point you're making Teetime. The ONS death figures are robust. They are the number of registered deaths in England and Wales for any given week. Scotland and NI have their own systems, but show similar trends.
There is a lag between the government's daily tally and the ONS because deaths aren't always registered on the day they occur. They also include all deaths, not just those who have tested positive for Covid-19 and have died in hospitals. The ONS figures, without any shadow of a doubt, are more accurate, despite being a week or so behind.
What is worrying is, as the FT quite rightly claims, is that the number of excess deaths is in thousands. Comparisons have been made with the last five years, comparing deaths on a weekly basis.
There is absolutely no doubt that the number of deaths where Covid-19 has been a significant factor has been underreported.
There are also some deaths, which did not have Cocid-19 on the certificate. The reason why non Covid-19 deaths have also increased is still unknown, but should not distract from the two main messages:
Overall deaths have increased beyond what would have been expected
Covid-19 deaths are higher than those the government has reported.