What do you mean not been tested? Are you saying everyone in the UK has to get tested regardless of symptoms or not?
A few days ago he said his neighbour was confirmed to have corona, which understandably would have caused him concern. But I will point out Rob, over here you can be "confirmed" to have coronavirus without being tested at the moment, due to the small number of tests available here during the last 2 weeks most people diagnosed had not been tested yet, so they might not be lying.
But hopefully over the next few days we'll have widespread testing, until then we can't know the asymptomatic rate, with some research from the University of Cambridge implying around half of the UK could have already been infected, on the theoretical assumption only 1 in 1000 people who are infected have symptoms. The only way we can test this model is widespread testing of almost everyone, so soon (Hopefully next few days) we will have hundreds of thousands of home tests being distributed across the country.
In reality, we know more than 1 in 1000 people have symptoms because more than 1 in 1000 people across the population of Lombardy have been hospitalised with coronavirus so far, and so half the UK wouldn't have already been infected as the theoretical model suggested, but even if it's 1 in 500 get distinguishable symptoms, or 1 in 250, it could mean a lot of us have already caught the virus and developed an immune response to it, and that earlier modelling is somewhat wrong.
Finding out that asymptomatic rate is the next major step to understanding the virus and our responses to it, so widespread testing South Korea style seems to be necessary.
EDIT: It's worth noting these home tests can detect if you've ever had coronavirus previously, not just if you're currently infected, they check for the anti-bodies your immune system develops in response to it. It's a little prick of the finger to test your blood like with insulin tests.