"Fair and reasonable"
Me again, since someone asked about "fair and reasonable".
Never mind my opinion, go look at the user ratings for AAISP and Zen (I mentioned them in the previous post), then go look at their tariffs. IDnet are also highly rated by their users, and priced similarly, maybe there are others too.
Their customers seem to think those are fair and reasonable combination of usage quotas, tariffs, and service quality.
What do these guys have in common? They're all focusing on customers prepared to pay for a decent quality of service, and their tariffs explicitly discourage mickeytaking. They are also ISPs who for better or worse get their national connectivity via BTwholesale, and that makes up a huge proportion of their bandwidth costs, and forms the narrowest (because most expensive) bit of their network.
You can go and look at their tariffs but I'll tell you now they're not the ISP for anyone who thinks 80GB in a day is reasonable, and (because they're largely BTwholesale-based) they're not necessarily the ISP of choice for anyone who wants ADSL2+ (which isn't yet widely available via BT, courtesy of their much delayed much overhyped 21CN rollout).
80GB of anything in a day isn't fair and reasonable if you're going to do it more than occasionally. If you want that quantity of data in a day more than occasionally you're not really welcome on any consumer broadband service, whatever verbage the ISP may have used when you bought it.
300GB a month off peak on a BTwholesale service makes zero sense (that's Enta and their resellers).
"for those of us willing to pay premium prices - say £50 / month - for properly maintained ADSL2+ service"
The LLU providers are largely on a race for the cheapest (as are many/most of their customers) so you might as well rule them out. The BTwholesale-based providers largely can't/won't offer ADSL2+ because the much overhyped 21CN is geographically restricted, late, and/or broken. You could try AAISP or (less likely as no routine 21CN yet afaik) Zen (yes, those two again).
It's a shame Metronet don't exist any more. They had an interesting business model and seemed able to deliver a decent QoS for their customers. But they too had tariffs where the more you use the more you pay. See a pattern here yet?