Do you mean we should assume the reality is about 150k people are getting infected every day?
Yep, something like that, we never really know for sure as we don't know exactly the proportion who are detected, but if you look at the ONS numbers of "estimated prevalence in the community" over a 2 week period and compare that to the positive tests, you see the number.
If we look at the recent ONS 1 in 50 - in England over a 14 day period, which is just over a million people infected during that time, because infections almost all last less than 14 days, that means a million needed to be infected, which if you compare it with the discovered cases, you'll see it's only around 20%.
Of course the numbers aren't directly comparable, due to sampling (and population) differences, but it's more that sort of ballpark than the idea that all cases are being picked up.