I wouldnt worry. ive been using around 150GB a month for over a year. Never been slowed down.
Last month i used nearly 400GB after getting an xbox one and again my download speeds havent changed
Thanks for that, didnt realise how quickly I'd go from 45gb a month to over 100gb in a wk lol
My average monthly usage is just under 300gb, you're fine downlading 100gb. The most I have ever done in one month is 700gb and I had no letter/warning from BT.