For reference for anyone else who runs into this a few notes that might help.
With Apple devices the actual device doesn't matter that much, it's the region associated with the apple id that is used on the device that makes the difference. Vodafone repeatedly asked me to change the region for my current apple id in order to download the app. Now you can do this but it will cancel your subscriptions, remove you from any family group etc. so for me that was never a good option.
However, I managed to find an old iphone that had been bought and used for years in the US which a) the kids hadn't destroyed and b) after connecting to a PC with iTunes I could get to factory restore back to its initial state.
From this point on I could setup the phone, select the UK region during the setup process etc. However, I'm not sure how significant that step was v.s the fact that I have a apple id that I setup for my parents that is registered in the UK region. So during the setup I signed in using their apple id and once it had finished the various steps I could now go to the app store and download the vodafone app since the phone/apple id it's using are registered in the UK region. All of the above I did while sitting here in the US.
All this because at some point over the last year or so vodafone appear to have taken away the ability to do the basic admin stuff on their website. Something that also appeared to have caught the vodafone support folks out since they kept asking me to look for a "continue on website" button which no longer appears to be there.
