It's a shame they don't do an unlimited package, especially considering the cost!
It would be interesting to see some stats on what the average Unlimited download is per month. Yes, AAISP does cost more; however, it does offer a 2000GB package with 50% unused data rollover, and the ability to flip usage contracts monthly.
For example, let us assume that my average monthly consumption was 400GB. I could buy 2000GB for £55 (including line) for month one, and then revert back to a 200GB contract for £10 less.
At the end of month 1, I would carry forward 1600/2 plus the monthly allowance of 200GB - or 1000GB for Month 2.
At the end of month 2, I would carry forward 600/2 plus the monthly allowance of 200GB - or 500GB for Month 3.
At the end of month 3, I would probably have to elect for the 2000GB contract for month 4 which with the carry forward would give c.2050GB.
Rinse and repeat.
Given the above, AAISP does not look to be that expensive - that said, its basic package is a bit frugal for a sizeable home.