I guess it is because the UK stopped doing as much testing. Presumably Google is reporting the number of positive tests (or something) and many people with covid don't bother getting tested any more.

The ONS is still doing random sampling and extrapolating to say how many people in the country are infected. "Around 1.7m people in the UK had coronavirus in the week ending 18 June" - https://www.bbc.co.uk/news/uk-51768274

You are misquoting, its estimated infections not cases. Not everyone who has an active infection gets a test and reports it, in fact its a small fraction.

Presumably it's based on the ONS estimates.