I think it is extremely important to gain some idea of the level of prevalence since that will place governments in a position where they can make decisions on a more informed basis.
Today's UK mumbers are about 17,000 confirmed infections with about 150 in critical care. It is known that the real number of cases is higher. What is not known is whether the real number of cases in the country is a little over 17,000 or 170,000 or 1,700,000 or 17,000,000. That degree of uncertainty renders highly suspect the basis on which any countermeasures are taken.
Why fliy almost blind when, without much difficulty, you can to illuminate the environment through which you are flying?
Prevalence is not very useful when dealing with a dynamic disease because it is a snapshot of the current. Even if starting today, by the time you finish testing even 1/100 of the UK population (660K) for COVID-19, the number of cases would have been very different to what is now. Bear in mind also all tests have a false negative rate. Let's say it is 10%. Are you going to fund the bill, not to mention the risk of crashing UK's radiology service, by performing 66000 CTs for confirmatory testing?
You vastly underestimated the work required to test on a population level. Just for a matter of perspectives-so far a little more than 100K COVID tests have been done in the UK. "without much difficulty", really?