The eSIM aspect here is irrelevant, it's an EE SIM.
In theory, you can port-in whilst abroad. But there are inherent risks, and if there are any issues, there is a good chance of you being left offline and unable to resolve until returning to the UK.
I would only advise doing so if you are fully prepared to accept the risk of losing service. Weigh that up against how time-critical the port-in is, and how much hardship is caused by waiting until you are in the UK.
