Are your numbers (80%, 150GB and 2TB) just random guesses, or close to reality?
The last Ofcom report on UK broadband (last month) showed that the top 10% of users (on ADSL2+ uncapped packages) used 50% of the total bandwidth usage, and the other 90% of users used the other 50%. Likewise, the bottom 50% of users used only 10%.
The report is here, and graph is on page 22.
The figure on page 20 suggests that the average is 30GB, again for unlimited ADSL2+ packages.
I *think* that means that the 90th percentile will be doing 30GB, the 10% heavy users will do more than 30GB on average, and the 50% lightest users will be 6GB or less.
I also recall seeing the presentation slides for a BT "fibre surgery". They suggested that average busy-hour backhaul usage figures could be applied for SFBB end users - IIRC it was 200kbps average. For 4 hours each evening for 30 days, that would be about 10GB.
I reckon your figures are a long way off reality - but I also reckon that knowing the actual figures is absolutely vital (to Plusnet) to understand whether they can afford to supply this package.
I suspect that their old package limit (of 250G for extra fibre) was so close to "unlimited" that it made no practical difference for *at least* the 90% level of users, and that the marketing use of "unlimited" will attract people onto both sides of that point - those who use less, but can't be bothered to calculate, and those who know (or believe) they use more.
From that, I reckon they will have to add some capacity, but certainly no more than double.
Anyway, Bob has reported (over on the Plusnet forums) that there is no FUP at all - which suggests that the bad boys won't be getting a warning letter, or issuing suspensions.