How is Tesco connected to PN? Are you thinking of John Lewis?
Sorry for delay. It was completely unrelated to PN, and more a 'compare and contrast' example.
PN (and a number of other ISPs) state fixed allowances, and may have 'unlimited' off-peak, while some other ISPs state 'unlimited' but
do have limits sometimes only found in small print, other times unwritten and hidden, but masked in terms like 'Fair Use Policy'.
Tesco says 'unlimited' but clearly isn't, and while their FAQ says one thing, and staff may back it up on the phone or via online chat, their T+C says another (namely that they can charge, though they don't get detailed enough to explain what fees could be charged, and the FAQ ignores this possibility).
For a customer / potential customer, I think it is a crazy situation where there are 'truly' unlimited ISPs and others saying 'unlimited' but meaning 'we say it for marketing reasons, but have our own view and if you go over our limit, we'll warn that you broke our FUP and require you to limit your usage'.
PN makes it possible for someone to 'go over' and pay for it, or limit (as low as nil) how much extra they are willing to spend. I'm considering a complaint to some body such as OFCOM to see if claims of 'unlimited' can be banned unless the ISP really means it. It might mean an ISP will create a new account offering unlimited, with a really high monthly charge, but should bring clarity for customers as to what
real world limits the ISPs have, whereas they say one thing but actually have hidden limits. Comparison sites often copy what they find, or use what they are told, without making any distinction about FUP limits vs unlimited accounts, all called 'unlimited'. Can understand that without the help/inside info from ISP staff, any unpublished limit is unclear, and if a customer is told they have exceeded a limit, there's no guarantee the same limit will apply the same day to any other customer of the same ISP, as an ISP might take note of previous instances when the customer went over an arbitrary limit and have a 'three strikes and you're out' policy or no actual policy and it depends on a whim, so which member of staff and what mood they are in that day might determine the action taken.
Back to the subject of charging for usage:
As another (dated, and academic) example of a charging method, Metronet (before PN took over) had accounts where a particular fee would provide a certain allowance. If that allowance was exceeded, there was a 'per MB' fee up to some upper monthly fee limit (but it seeemed to me to not have a restriction on traffic). I remember one account having a maximum monthly fee of approx 100 pounds, but it was probably competitive for a business if the alternative was a leased line and the usage levels could probably have been significantly higher at 100-odd quid (at up to 8 Mbps download speed on ADSL) than a 2 Mbps leased line...