It seems like we can use the testing confirmation rate as an upper bound on the fraction of a country that has COVID-19, at least early in a pandemic.
The day that FT article was published saying 50% of the UK 'might' have had C19, the confirmation rate among the sickest patients with symptoms most matching C19 — the only ones qualifying for a test — was 22%!
Of course the real fraction of the population with COVID19 is surely far far lower than among sick patients referred for a test. I'd guess the rate among the whole population was more like 1% at the time.
(In theory lots of people could have gotten the virus and cleared it in February so would testing negative now, but this makes literally zero sense when combined with other data like its spread rate, the fact that confirmation rates were much lower at that time, how diseases spread exponentially, hospitalization rates now vs then, etc.)
The chart above is taken from : http://epidemicforecasting.org/
You can take a look at estimates from their epidemiological models for various countries including your own.
Meanwhile, Imperial College London have also modelled estimates of the percentage total of population infected for various countries.
Check out their report here : https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-Europe-estimates-and-NPI-impact-30-03-2020.pdf