T Mobile don't allow tethering on their unlimited plans so that rules that out. 
Our Full Monty plans will give you unlimited use of our internet on your phone service when you're in the UK. This means you can use as many MB/GB as you like and no fair use policy will apply. Please note, internet on your phone excludes using your phone as a modem (tethering) and network traffic controls apply.
Bottom line, they won't miss a trick to get more money from you.

As mentioned, I have enabled WiFi hotspot tethering on Vodafone PAYG (Samsung Galaxy Fame) and it worked OK on 3G at least. I'd pre-purchased a data 'freebie' and it worked all last weekend, and cost me nothing more than my 'freebie', even though it theoretically should not have worked at all.
I'm not suggesting you place any dependence on that. It's hard for the providers to know you are tethering as it basically looks to them just like any other data, but I suspect Vodafone would find out and block it if I made it a habit. But if have an unlocked phone you might consider taking a punt on a Vodafone PAYG SIM, just for experiments. If tethering works (not guaranteed) it might work long enough for you to evaluate the technology and see if it lives up to expecations, without signing up to any contracts?