Roaming charges are not created equal. Some carriers charge a flat £2/day for European roaming and it's perfectly reasonable. Others charge £8/day with a daily data cap and per-minute call rates on top. The gap between "fine" and "highway robbery" is enormous depending on who your carrier is.
Here's a real comparison across common travel scenarios.
A week in Europe
UK carrier roaming (mid-tier plan): EE charges £2/day for EU roaming on most plans, capped at 25 days/year. For seven days: £14. O2 is similar at £2/day. That's actually competitive with an eSIM for short trips.
UK carrier roaming (basic/older plan): Some older or SIM-only plans charge £5-8/day. Seven days at £5/day is £35. At £8/day it's £56. An eSIM with 5GB for a week costs roughly £7-12.
Verdict for Europe: if you're on a modern UK plan with £2/day EU roaming included, the eSIM saving is minimal. If you're on an older plan or a carrier that charges more, eSIM saves you £20-40 on a standard holiday.
Two weeks in Japan
UK carrier roaming: Most UK carriers charge £5-8/day for Japan. Fourteen days at £5/day is £70. Some carriers also cap daily data at 500MB-1GB when roaming, after which speeds throttle severely.
Japan eSIM: A 10GB plan for 15 days costs roughly £15-25 depending on the provider. Call savings: you don't make calls on an eSIM data plan (data only), so use WhatsApp for voice.
Saving: easily £50-55 on a two-week Japan trip. Possibly more depending on your carrier.
Ten days in the USA
UK carrier roaming: The US is expensive for roaming from UK networks. EE charges £5/day. Ten days is £50. Data is often capped or throttled at 500MB/day.
USA eSIM: A 10GB/15-day plan costs around £15-20. Saving: £30-35 on a ten-day US trip, plus you're not dealing with throttled data.
The hidden roaming costs people miss
Per-minute call charges. Even on daily-rate roaming plans, making a call to a local number abroad often triggers per-minute charges on top of the daily rate. Using WhatsApp for calls eliminates this.
Data throttling. Some carriers advertise "unlimited roaming data" but throttle to 2G speeds after a daily cap. 2G is genuinely unusable for modern apps. The throttled plan costs the same but delivers a fraction of the value.
Receiving calls. Depending on your carrier, receiving a phone call from abroad can trigger roaming charges even if you're not making calls. Enable Wi-Fi calling and have people reach you that way.
The overall picture
eSIM wins clearly over roaming for: Japan, USA, Australia, any non-EU destination, and anyone on an older or pricier roaming plan. For EU travel on modern UK plans with cheap daily roaming, the financial case is weaker - it's more about convenience than cost.