Short answer: A prepaid UK eSIM with fixed GB and EUR checkout beats international roaming for most visitors to London, Edinburgh and business trips across Britain.
How much data for the UK?
5 GB for a long weekend in London; 10–15 GB for a week including Scotland or the coast. Use Citymapper, TfL, Uber and maps without Wi‑Fi hunting.
UK eSIM vs Airalo / Holafly
Major providers all sell UK data — the difference is checkout experience, support and total price. See Zenora Esim vs Airalo and vs Holafly.