Dual SIM iPhone SE third edition

Re: “… having taken all steps to ensure there would be none …”

Unless you turned “off” your UK eSIM line (From within: Settings -> Mobile -> eSIMs)

— and something which you didn’t mention one way or the other in your post ??? —

Axel’s hypothesis seems a very plausible explanation.

Additionally, Setting a PIN on your eSIM(s) is another way to positively ensure that any particular line remains disabled w/o your explicit approval.

Use a SIM PIN for your iPhone or iPad - Apple Support

It appears that the UK service provider (eSIM) charged you for roaming. There must have been a service drop in NZ for a brief moment (152Kbytes can be consumed within seconds) and your UK (eSIM) line must have picked up the slack as a roaming service. This is very difficult to explain.

Your best bet is to question the cellular service provider that charged you (UK eSIM) and ask them for a refund.

Axel F.

As users....just like you, we cannot speak for your UK carrier, but I would expect that they have good documentation. You will have to ask your UK carrier about this.

Our opinion..... you may likely spend a good deal more than 7 pounds worth of your time trying to get a refund, but it's your decision.