Hi Steve,
Thank you for your interest and ideas. Yes, I'll need to use Vodaphone because all the others don't necessarily provide decent coverage and the current owners of the house have done the same research and come to the same conclusions. There is a Vodaphone mast that seems to give pretty good coverage of the area and my idea is to stick an external antenna on one of the chimneys and point it at the mast. I should achieve line of sight by doing this and, with the Poynting 11dB gain antenna, I'm hoping this will ensure I get a decent signal to the router.
There is no 5G coverage at the moment but my thinking is to at least future-proof a bit by (a)buying the antenna mentioned above, which will be 5G capable and (b)buying a 5G capable router (hopefully). Until this point I didn't think it was worth paying the extra for a 5G data plan when I can only get 4G as a maximum but the plans you've suggested have an unlimited data 5G plan which is basically the same price as the Vodaphone unlimited 4G plan.
Having done more digging I am much, much less concerned (tending towards zero) about the ATOM station data use, as the unlimited plans are on a par with what I'm paying for Sky broadband at the moment, so I have no compunction in transferring to one of those. If the VOXI 5G plan only gives me 4G but a bit of future proofing then I think it's worth going for that, so thank you for your suggestion.