Just to clarify:
-- an ISP refers to an Internet Service Provider, and this typically involves some kind of land-line connectivity (cabling running into your home, coax or phone line) and then a modem/router, or modem and router, that provides you with online access (via Ethernet cables and WiFi).
-- a cellular carrier involves wireless connectivity only, and is supplied and maintained by your chosen carrier's cellular network. This involves cellular radio signals that are emitted by nearby towers and antennas mounted on other structures, like a tall building.
-- a VPN is a Virtual Private Network service. It is NOT going provide you with any online access, you still need an account with an ISP and/or carrier. A VPN is simply a more secure way to connect within whichever online connection you're using. So while you can get to the Internet using either a broadband connection (ISP) or mobile data (cellular) with or without a VPN service, you cannot get any access to the Internet using just a VPN service.
So instead of focusing so much on looking for a free VPN service, you should probably be looking for a free or low-cost ISP or carrier service instead. Keep in mind that it costs money to provide such services so that's why they typically involve a monthly fee, or limitations (reduced bandwidth speed or data caps) with increased user data mining when it involves free plans. Here in the U.S. there's a free government-supplied service (Lifeline) and some corporate, also limited free services (FreedomPop) but I don't know what the status is in the U.K. Given the spread of fascism I won't be surprised if Lifeline soon disappears.