This is a big problem; it's pretty much impossible for any ISP to have enough capacity for all their customers to be running flat out at the same time. They rely on the fact that most people's traffic is a bit stop-start - load a web page, the line sits idle while reading, then load another, and so on.
That just about got them through to start with, and they were happy to throw money at marketing and boasting cheapest ever deals to get punters through the door, rather than at investing in support or actual internet infrastructure.
Over the years though, thanks to YouTube, iPlayer, and now things like Netflix and Amazon Prime, that model has changed, to the degree where a substantial number of their customers aren't doing stop-start traffic, but are streaming.
When that started to bite was when some ISPs decided to try and make a noise about the BBC having to pay them for traffic. And it's caused real problems for some who chased the budget market; I suspect it's what put paid to some of those who fell by the wayside over the years - the investment required to catch up and cope with modern traffic patterns just became too large for them.
Various fudges have made the crunch perhaps a little less awful than it could have been, and modern codecs mean you can stream video in much less than the capacity of an average broadband connection now, so that leaves wiggle room.
However, there does seem to be a fairly big difference in terms of capacity management these days; smaller outfits will try to do their best and not be the bottleneck, as much as they can. Larger ones will do what they think they can get away with, and try to avoid mentioning it, or waffle about 'network management' and 'traffic shaping' if you really try to get them to say something.
Twelve month contracts are the norm; with loss leader three month periods, and options to grab money via domestic phone calls, line rental and so on, they probably do think it's worth the money to pay a few people (probably on min. wage) to stand around and sign folk up. Give it a while, and it wouldn't surprise me to see some big ISPs trying to shift up to 18 month contracts, just as we've seen with phones.