The latest UK broadband report from Which magazine has slammed BT, Plusnet, TalkTalk, and other British ISPs for failing to deliver the "up to" speeds that are advertised on their websites. BT and Plusnet were two of the most egregious offenders: for their "up to" 76Mbps Internet access package, only 1 percent of customers actually got the advertised speed. Virgin Media, you might be unsurprised to learn, put in a much better showing than the other mainstream ISPs: 98 percent of Virgin customers got the advertised speed.

The Which report (PDF) is based on data gathered by Ofcom in November 2014 and published in February 2015 ("UK fixed-line broadband performance, November 2014.") In turn, Ofcom used data produced by SamKnows, which placed Whiteboxes in around 2,000 UK homes to poll their connection performance. 2,000 homes is probably a bit on the low side, but Which maintains that it had a "sufficient sample size" with which to draw its conclusions.

In the UK, advertising is regulated by the Advertising Standards Authority (ASA) and enforced by CAP and BCAP. The current policy on advertising Internet access speeds in the UK is:

Where advertisers make a numerical speed claim that is likely to be understood by consumers as the maximum speed of their service, they should be able to demonstrate that the speed is achievable for at least 10% of the relevant customer base.

So, for example, at least 10 percent of BT Infinity 2 users should get download speeds of up to 76Mbps, and 10% of TalkTalk ADSL2 customers should get up to 17Mbps—but, alas, they do not.

Which's report makes for pretty damning reading, but here's a summary:

Nationwide, about 17 percent of fixed-line broadband users actually get the stated "up to" download speeds, falling to 15 percent at peak time. (This figure is inflated by Virgin's superior network, though; less than 10 percent of ADSL/VDSL users receive the "up to" speed.)

On average, the maximum speed received by UK households was only 68 percent of the advertised speeds.

Unsurprisingly, the figures are much better in urban areas (around 30 percent getting advertised speeds), and awful in rural areas (under 5 percent).

Virgin Media, by virtue of using its own cable network rather than Openreach's last-mile copper-wire network, is much better at hitting advertised speeds. (The average max speed was actually faster than advertised for Virgin customers.)

In general, the higher the advertised "up to" speed, the smaller the number of people who actually hit it (94 percent of Virgin customers on the 50Mbps package actually got 50Mbps; only 20 percent on the 152Mbps package got 152Mbps).

Overall, the main takeaway is that ADSL and FTTC (i.e. "superfast fibre broadband") providers need to be more accurate with their "up to" speeds, especially in the case of rural customers. It's also clear that Virgin, because it owns and operates a much newer last-mile access network, is capable of providing more reliable speeds.

BT, Plusnet, and TalkTalk all disputed Ofcom's report when it was published in February, claiming that they do stick to the ASA/CAP/BCAP guidelines, but Ofcom seemed unmoved.

Perhaps the most shocking result is the dearth of customers who actually get 76Mbps. Technologically, we know that VDSL2 (the technology that underpins Openreach's FTTC/Superfast product) deteriorates very rapidly when you're more than 200-300 meters away from the cabinet. We know that 76Mbps is probably only attainable by a small number of households that are close to a street-level cabinet and have a good-condition telephone line—and yet ISPs continue to advertise the product as "up to" 76Mbps.

Over the next few months, new Ofcom regulation will make it much easier for UK customers to leave ISPs if their connections are really bad—but for now, there's nothing you can do if your connection speed is merely misadvertised.