Andrews and Arnold were my choice, but their Three SIM card does not work at my address.
Almost no mobile provider will provide a static IP or even an inbound routable dynamic, most are CGNAT or firewalled if you get IPv6 to protect majority of customers.
Get any workable service, and use a router that can handle L2TP with AAISP’s service:
https://www.aa.net.uk/broadband/l2tp-service/
If you can’t get Three signal that means Smarty and ID are out; so you need to see which of Vodafone, O2 or EE gives you the best data rate. If Vodafone look at Voxi, if O2, look at GiffGaff, and if EE then look at 1pMobile for short term reasonable cost.
25 years of broadband connectivity since Sep 1999 trial - Live BQM