Building on that, you've got two rigs. Simplest is the phone itself: drop a UK SIM in, get on mobile data, install Termux, and run 3proxy bound to the cellular interface. That phone is now your SOCKS or HTTP exit. The EC20 route is cleaner but wants a host, so drive it from a Raspberry Pi or mini PC over USB. Set the APN, it comes up as a network interface, then bind 3proxy or dante to it. Either way the SIM's data is your exit IP.
The catch: UK carriers run CGNAT, so your SIM has no public inbound IP and you can't port forward to it. Keep the proxy and its consumer on the same LAN, or reach it from outside with an SSH reverse tunnel through a cheap VPS. For fresh IPs, toggle airplane mode or send the EC20 an AT command to reattach.
What keeps accounts alive is discipline, not the rig. One account or very few per SIM, keep each sticky to its own IP, hold timezone and locale steady, no IP jumps mid session. Honestly it's cheap for a handful of accounts but fiddly at scale, so paid rotating proxies start paying off once you grow.
Best,
Floqal