"On the edge of one of the most commercially intensive towns in Hertfordshire (UK), which is hardly "rural" we have a 30Mbps VDSL account that delivers about 6Mbps at best (sometimes down to 1Mbps)."
Full disclosure: I am an escalations manager for an ISP.
If you have 30Mbps sync then you should get close to 30Mbps over that line. However....
There can be line issues which don't affect sync speed, but do affect throughput usually visible from the router as a high rate of errors. And yet it's amazing how often I will encounter end users who complain that their line has always been slow, but they've never actually had an engineer investigate the line. Sometimes this is because the customer has never reported the issue to their ISP, sometimes it's because the ISP has offered an engineer but advised the customer that the visit may be chargeable if no fault is found and the customer decides just to put up with the issue.
With FTTC the old "hot VP" problem has pretty much been eliminated. Openreach actually did a decent job of capacity planning. So contention at a cabinet level is very seldom a problem. Which means if there are any contention issues not actually caused by the line then they tend to be beyond the Openreach handover point. It's hard, if not impossible to test this however if your ISP has their own speed test servers run a test to them and then to another speed test server off the ISPs network. If there is a discrepancy then you're probably looking at an issue on your ISPs network rather than on Openreach's network.
So if there's no performance issues up to the OHP then it's your ISP who are to blame. But even then I tend to find that the problem is more likely to be a customer premise issue.
So far this year I have dispatched engineers to customers where no performance issue has been found on the network at all. In some cases the engineer could get perfectly good performance connecting to the customer router with his phone. In other words it's the customer's devices or often a single device that is causing the problem. In other cases the engineer found poor wifi performance often due to interference. Which is why ISPs always ask that you run performance tests wired to the router wherever possible, but it's amazing how many people tell you that they are wired when they're not or who tell you the performance issue exists even when they are standing right next to the router and when the engineer attends they find the issue only exists in the attic while the router is in the basement. Sometimes wifi problems are caused by the end user messing with the router config and a quick factory reset mysteriously resolves the fault. Then there are the people who have bought their own "high performance" router and reverting to the ISP supplied router resolves the fault.
The worst thing about all the End User Faults is that ISPs usually make sure that they have asked all the right questions and the users have claimed that they have done all their own testing and they are 100% sure it's a network fault. Then the poor old ISP gets shouted at by the user when the engineer returns the job RWT and the end user gets charged.
Yes there are problems with the Openreach network, but they can usually be fixed by a simple fault report. There are however many more issues on customer premises and ISP networks, but Openreach get the blame. Especially by unscrupulous low rent ISPs. These are often the ones who don't even by directly from Openreach but resell a larger ISPs services.
All of the above explains why I am seldom sympathetic when people tell me their slow broadband stories.