I don't see why broadband connections aren't 'metered' like other connections. You should pay for what you receive, not for some notional package which is unattainable. If I'm on a 300Mbps contract, but only get 100Mbps then I should pay the rate for 100Mbps.
The ISP's get let off on the basis their service is "up to 300Mbps", but a gas supplier wouldn't get away with a service offering gas "up to 100 kWhs a month" at a fixed price and then only giving you half that. If we got charged on a consumption basis suppliers might then make more effort to see their connections were actually capable of delivering what they promised.