After asking AOL and checking on my Nan's AOL account at keyword BROADBAND, I can confirm that there is no cap and they do not plan to introduce a cap. you can read their "conditions of service" at
http://www.aol.co.uk/cos.
I was pleasantly surprised at the politeness and knowledge of the rep I spoke to knew strait away what I was talking about and said that even though I have BB with Ntl I would need a visit to swap the modem over to an AOL owned one but other than that I could change asap if I wanted to, and I am seriously considering it. For £2 a month more it may well be worth it.
Well done AOL, you still are great, just how I remember you.
edit If the above link doesn't work for you try :
http://www.aol.co.uk/about/legal/cos.html