eSIM is easy for all the carriers that use them (all the main ones). On O2 they still needed to send me a bit of paper with the QR code on or go into store for it - seems crazy!
Vodafone is much easier - and any time you delete the sim (swapping phones etc) they straight away email you a new one and you’re ready to go.
Something to note, you are liable for import duty and VAT when you bring the device back to the UK. If for any reason you forget to fill in that on your paperwork the new box and cable can help remind HMRC as you go through…… if you happen to keep it.
The bit that makes this tricky is that it’s all tied to an email ID.
Not as simple as selling some hardware or a game cart - you can’t wipe it as the activation servers won’t recognise it, so it’s ‘stuck’ with an Apple ID that’s probably tied to loads of other stuff.