Testing gives an initial view on the picture. We've seen how Italy took off first, then Spain, and France third. When you have in France 10-15k tested with 4500 positive cases and 80 dead and UK has ~30k tested with 800 cases and 10 dead you can see the countries are in different stages. Spain Italy France have called lockdowns on around 100-200 dead. Whatever UK do, we're not there yet as it happens to even copy Europe, 2-4 weeks off.
After that it's about testing the people in serious condition. When you have populations of 10s of millions, 50k testing, 120k testing it doesn't equate to much and testing 500k is a huge manpower undertaking and contamination risk, waste of time for those doing the lab testing. Attention and testing needs to be on the people who come in the hospital and those who are trying treat the patients. You have to be retested and be safe to treat people. There's no point taking peoples results if they have mild symptoms now.