Smart Meters call home using the DCC network.
https://www.smartdcc.co.uk/( The DCC was set up by the gubbinsment to centralise smart-meter communications to overcome the old proprietery systems operated by individual energy providers, in order to help make switching suppliers work whilst maintaining the Smart capability.)
They contract out the actual comms to two suppliers, one in the North, the other in the Central and Southern Communication Areas:
https://www.smartme.co.uk/smets-2.htmlIn the North, Arquiva (the same company that runs the UHF TV transmitters) uses the Long-Range Radio system, which is UHF at frequencies 412-414 MHz and 453-455 MHz. The Smart Meter Tx has an output power of 1W. Here's an example of the kind of thing that's being used (P7 gives the WAN spec):
https://www.edmi-meters.com/europe/wp-content/uploads/2019/05/EDMI-Variant-450-DB-Comms-Hub-User-Manual-v1.2.pdf(The Dual-Band that's mentioned refers to the Home-Area Network, which can operate at 2 different frequencies to overcome range issues due to thick walls etc. Nothing to do with the Wide Area Network.)
I don't know if Arquiva are running this entirely out of their existing TV UHF sites, or if they have added infill masts.
But I think your UHF TV coverage will be a better guide than 3G coverage.
In the South, Telefonica (O2) provide the comms over 2G/3G.
In both cases, the meters operate in a Mesh mode, so that if one is out of range of the main infrastructure, it can pass it via others in the mesh.