@gizzblue out of interest where did you find that info?
I've been looking at the unlimited deal for a member of our family and the only details I could find about tethering were this:-
You can tether and hot-spot all of your data with no speed restrictions
on their plans page here:-
Edit to add,the only FUP I could find with the unlimited data plan was that you can only tether 19GB a month if roaming in the EU.
I found that info in the unlimited plan section of their price guide here but no details of a tethering restriction in the UK:-