I used the network monitor in the Linux Mint version of "task manager" to monitor my hungriest machine for a day during which I played much Youtube music videos. It seems that on a good day I can use 1.2GB of data. I guess 40GB a month should cover all of our needs.
Going landlineless is not quite an option, yet, we are just outside of the "good indoor coverage" of both 3 and EE 4G networks. A mate who lives closer to the centre of Reading than we are uses his mobile phone on 3 to get sustained downloads in the 20Mbs, or about 3 times the rate we get down our copper based connection. It will be getting here eventually, then I will look again at the options.
Rans6.....