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I use https://esimdb.com/uk instead of a single app.

It lists the cheapest e-SIMs for travelling to each country.

Yeah this is the best option by far.

Hmm... that's 5x as expensive as getting a regular SIM, though :/

I've used Nomad (https://www.getnomad.app/), they usually have a 10-25% code somewhere. I've had no issues and used them 10s of times.