I've always (perhaps naively) assumed that's exactly what Andrews and Arnold do do, because of their published packet loss and congestion reports, their we are not the bottleneck commitment, the fact that they charge plenty for usage and don't offer all-you-can-eat infinite free deals like many, many ISPs do. They presumably don't make a loss because they charge the customers more and they charge them for what they are actually doing. AA now get bulk purchase deals from TalkTalk Wholesale so they can do their various 1 TB and 10 TB pre-booked per month deals. RevK the boss has talked quite a bit on occasions about costs of purchasing bandwidth or traffic.
I've never ever seen the network slow down, not even for example during the Wimbledon men's final which I watched live streaming because I felt too ill to go downstairs and watch it on TV (via Sky dish). And even then it didn't wilt. I know that they buy extra bandwidth from BT et al every year before Wimbledon is going to be on.