I'll try again and hope I don't get the spam message.
On PAYG, you can buy what they call an Add-On for fifteen pounds, expires after 30 days....that's the one I referred to above. There are other types of Add-On too. At the end of each month you can choose any type of Add-On or revert to the basic non-expiring PAYG at 3p/min, 2p/text, 1p/Mb. The flexibility allows me to use several phones and decide which one I want to use unlimited data on for any particular month. It works out surprisingly cheap.
Three also offer SIM-only contracts offering unlimited data, the cheapest of which I believe is fifteen pounds a month, and includes 4Gb of tethering. All unlimited data deals, PAYG or contract, include 4G where available, at no extra cost.
I've always used Three and never regretted it.