Find the best Mobile Phone signal from one of the four carriers who are; Vodaphone, EE, O2 and Three.
Do this by checking with friends/colleagues who live ‘next door’ or via the online cover maps;
Vodaphone Cover
O2
EE
Three
Remember they are only predictions. Some will give an indication of indoor and outdoor signal quality.
IF (and I mean IF) one of them tells you there is good indoor 4G or 5G cover, then mobile broadband is the way to go. Either tether to a phone or get a 5G router and insert a SIM card from the relevant provider and create a local Wifi network.
To save money remember that all four providers are the most expensive way if you buy direct. Once yo know the best network, go to this Wiki Page to find to who resells the provider that gives the best signal.
NOTE Not all resellers include the fastest options (i.e TalkMobile who resell Vodaphone, do not support 5g yet)
Finally, you need to understand how much data she will be using which is difficult to predict to start with. VOXI, GiffGaff and Smarty all offer monthly contracts with a lot of data at a very competitive price. So if it is suitable, pick one of those and then tweak the data allowance over the first couple of months until you get the balance between price and allowance right.