I think your best / cheapest option would be to take the prepaid 1 year SIM at £48 which works out at only £4 a month, and then purchase a non recurring boost/bundle if/when required e.g. £25 for unlimited or £18 for 200GB

In reply to a post by Gari:

I think your best / cheapest option would be to take the prepaid 1 year SIM at £48 which works out at only £4 a month, and then purchase a non recurring boost/bundle if/when required e.g. £25 for unlimited or £18 for 200GB

This is exactly what I've done. Sim card arrived today, plugged in and tested.

How many users are relying on the internet when it fails over? If it only needs to work when you're in the house then a router that connects to your phone hotspot over Wi-Fi, or can tether via USB would be a workable option.