According to AA website - "OFCOM produce reports on UK broadband activities and in 2014, on average, households download 51GB per month". It's 2016 and internet world has moved on.
With 2016 video streaming services that are getting more and more popular, or even simple Sony PS4 game updates, 100GB is simply not enough.
Netflix as per https://help.netflix.com/en/node/87 says - "High (Best video quality, up to 3 GB per hour for HD and 7 GB per hour for Ultra HD)" - this means if you have Full HD TV you can actually watch around 14hrs of content per month to hit 100GB.
Last Bungie Destiny update was 17GB in size. If you want to buy game online it's ~ 40GB download.
I'm on 200GB FTTC tariff and very often I need to top up. I appreciate the 1TB service, but not everyone can get it (12 months commit non-transferable). I watched a single season of series from Amazon Prime in February an it used over 80GB of my monthly allowance.
I'm sure AA is familiar with Moore's law - http://phys.org/news/2009-01-internet-growth-law.html.
I propose you follow the Moore's law for your Home::1 allowance and simply double it every 2 years.