Granted it's population is small, but Iceland did some testing recently which indicated roughly 5% of the population was infected, with half of them showing no symptoms.
I have little knowledge of the demographics of Iceland, but suspect they'd have an average age below ours and their population would be heavily waited to the capital, so wouldn't expect those numbers to directly translate to the UK. But that % is a fair bit higher than what most experts expect ours to be ( <1% ), so it's important to know where we stand, so we can plug that figure back into the models.
The perfect scenario is everyone gets a test and we’re all locked up for a few weeks and they treat those with it.