Those totals are the reported cases - there's a big difference between actual cases and reported ones..
Roughly half of those people getting Covid are believed to be unaware of it, or get such mild symptoms they don't think much about it.
Informal research (by me..) indicates that around half the population would not bother to test if they got moderate symptoms. As a follow up question, well over half told me that they would not report being infected if they had a positive test. The NHS 'pinging' system does not seem to enthuse people - they'd rather hunker down until they got better, than become a statistic.