To go over 100gb a month, someone (probably you) is downloading a HELL of a lot!
No amount of gaming or iplayer usage would send you anywhere near 100gb.
Sounds to me like you a torrenting like there is no tomorrow!
Are you serious, posts like this really annoy me, I go over BT's cap of a 100gb all the time and I hardly ever download anything illegal bar the odd TV serial, most of my usage is taken up with iPlayer and 4OD, they are my only TV, I do not use a conventional television any-more, a 1 hour (non HD) television programme on iPlayer for example is around 600/700mb, x 10 that 6/7gb already with just ten programmes and I watch a lot more than just ten programmes a month and that's not even including the HD content.
Also take into account a HD movie rental via 360 or PS3, 10gb, yes 10gb, for just one movie wipes out 10% of your monthly bandwidth, then there is youtube which now eats bandwidth due to most videos now using HD, also I personally stream 192kbs trance radio stations off of shoutcast most of the month and that also adds up to a hell of a lot, plus Steam, I can't even finish downloading all my purchased games yet due to the size of them.
Also don't forget about the bandwidth the PS3 and the 360 network uses, downloading games/demo's, wireless laptops, and we've not even taken into account just generally browsing the web, it all adds up.
Seriously it's very very easy to legitimately go over 100gb if you use your PC as your main hub for media and entertainment rather than something you just surf on and play a few games on in the evening.
I'm sorry to rant on but it just bugs me that some people automatically assume that you are a pirate just because you burn through 100gb, for me 100gb is nothing, absolutely nothing, and things are only going to get worse when more TV services and media transfer on-line.