It uses open-meteo.com. Their forecast API returns data from around 30 different models depending on location, and typically uses Met Office data in the UK. I don’t force to only Met Office though, as they don’t give data for everything shown (such as the pollen count per species).

Open-meteo has a very generous free tier, and reasonable pricing. It also lets you fetch weather from previous days, which I’ve included so you can see if it’s relatively hotter/colder etc. than yesterday.