Don't forget that AAISP have a roll-over policy which carries forward half of any surplus quota to the next month and that you can also change the quota tagged to your account at any time.
If you use 400GB in an average month you could sign up to the 2000GB package in month one then drop down to the 300GB package in month two. Over the months you would see the following allowances:
Month 1 Allowance 2000GB Usage 400GB Carried forward 800GB
Month 2 Allowance 1100GB Usage 400GB Carried forward 350GB
Month 3 Allowance 650GB Usage 400GB Carried forward 125GB
Month 4 Allowance 425GB Usage 400GB Carried forward 12GB
At that point you again take on one month of the 2000GB quota.
Rounding things out your annual spend would be 9 x £45 + 3 x £55 = £570 or an average spend of £47.50 per month which again is not an unreasonable amount given the service level.
Not only does unused quota roll over, so does unused roll-over. (Not easily obvious from your
correct example

). On my 200GB I started most months with well over 350GB available.
Apply the same principle to the 1TB or 2TB tariffs and .....
Plus they keep improving the tariffs. I joined on 100GB 80/20. After a while they dropped the 40/10 product and gave those users the full 80/20 at the same price. (Obviously that only benefitted those who could get over 40Mbps sync). At the same time they dropped the price of 80/20 to the price of 40/10.
Later they doubled the allowance on all tariffs.
Whether there have been any later improvements I don't know. I ditched landline broadband in December 2018.
It's Max talking rubbish again for the sake of something to say.
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Connections: OnePlus 8 Pro max 165Mbps down, 24Mbps up on Three, and B311 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
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To argue with mindless bigots is foolish.