I buy oat milk, but I assume the kind that lasts a week. UHT is not very popular on these islands.

Ordinary, but skimmed milk - mumsnet says you need to boil it for ~10 mins and then let it cool to ~40C before adding the live yoghurt culture. Working well so far!

UHT is usable, and doesn't need to be boiled, but for that added convenience it costs nearly twice as much.

Fresh milk is currently £1.45 for 2.27 litres, down from £1.65 at peak.

Tip for UK readers: You can put an unopened 4pt plastic milk bottle in the freezer and it will keep as long as UHT. Defrosts in a few days in a fridge. (Was handy during COVID lockdowns)

The latter, UHT isn't popular at all here in the UK, unlike on the continent.