eSim activation abroad

Hi

I'm off to Canada over Christmas and I've bought my children 'new' IPhones. These are eSim compatible. I was planning on giving them whilst there, is it possible to activate an eSim abroad, or do you need to be within the UK for it to register correctly? They are already on Smarty but the new phones don't have physical sim slots. I've looked through the faq and posts but can't find any definitive answer. 

Thanks