For anyone else not from the UK, ONS seems to be some official body doing some sort of, well, sampling.

Do you have a link to their current stats? I found this page[1] but it's not very accessible, not sure this is what you mean.

[1] https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...

This is a more friendly presentation of the same data, though note there’s a lag in the ONS data as it’s from ongoing sampling rather than self reported tests. At time of writing this is up to 30 July (next update 6 August):

https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...

Does this help? https://coronavirus.data.gov.uk/