Thanks for all the helpful suggestions, just thought I'd share a quick update as to where we are now:
• Sadly it seems like it's important this phone can make decent quality calls as well as have a good data connection. I'd personally have been fine with an option like spusu (in fact I signed up to their £1 for 5 GB offer to try them out and got great 5G speeds), but as it's not me making all these decisions sadly it looks like providers on a network without 3G (Vodafone/EE right now) that have no VOLTE support are now out of the question (Now Mobile, RWG, Talk Home), and similarly that providers that have no HD voice support whatsoever (e.g. spusu) are also off the list.
• CMLink looked like a great deal to me but unfortunately it's looking like the relative in question would rather not be getting their mobile services from a Chinese company who's website is partly in Chinese. If it was up to me I'd likely go for this as it's on the EE Core network just like other MVNO's like 1P and it has HD voice, VOLTE and VO WiFi as well so would be almost perfect for this use case. But sadly I think getting them to tick the "I'm happy for my data to be transferred to China" check box would prove difficult. However I'll keep it in mind if a use case for it comes up in the future.
• So I'm starting to look again at 1P dispite the minimum spend requirement. Is it as simple as £10 every 3 months, or is there also a requirement of £2.50 minimum every month with a fee being deducted if you fail to meet this?
• I'm also now starting to look again at iD, although I think that's less likely as Three is very weak and slow in the location, although it does still get data throughput there (albeit only about 3 MBPS) so is just about also a possibility. But it does have VOLTE, VO WiFi and the 1p/MB rate hence why I'm considering it.
I appreciate that finding the 'perfect offer' is getting pretty hard now, but please do post any PAYG offers you think might suit, particularly those that are:
• On EE or vodafone or one of their MVNO's
• Support HD voice, VO WiFi and VOLTE
• Have a relatively low per-MB rate (10p per MB is too high)
Or maybe alternatively we could get a monthly contract with a very low price. I looked at Lebara via MSE and this looks good in the medium term but I'd need to be re-doing this when the promotional rate expired and would like this to run with little or no intervention once set up if possible.
Again, I've really appreciated all the help on this so far.