See comment from another UK resident further down. I suspect it depends on the contract you have, and quite possibly when the contract started.

It appears to be something to do with using PAYG SIMs for mobile broadband. Back when I lived ten minutes from one of the largest cities in the country I used 4G, but didn't run into this or their CGNAT crap because I tunneled out to a sane ISP.

Given that you can buy a SIM that'll give you a couple of hundred GB of data for under a tenner, it seems reasonable that they'd block stuff you didn't want young children getting access to (easily).