The ONS surveys and Zoe have consistently suggested around 20% of cases are picked up as positive tests, which would put the 7 million up to 35 million, and that excludes the numbers in the first wave where the ratio was nothing like that. But as alreadytaken says, there are also re-infections of perhaps 1-2% even within that 7 million.
Flu, despite vaccination, significant prior immunity and less infectious than covid infects up to 20%, and RSV almost certainly more, average 6 "common colds" a year I believe - the novel nature of coronavirus has just compressed this into everyone getting it in a short time.