they are "at most" some speed. What even is the point of writting such spec?
the car analogy would be if you measure some car going down a hill (doesn't even matter if tumbling and killing everyone inside), measuring that it was 300mph and then advertising it as "speeds of 300mph!!!"
And it's not like that everywhere, the UK has started what I hope develops into a trend: https://www.bt.com/broadband/deals/
What you'll actually get is determined by factors entirely out of their control. Whether that's an install engineer using a reel of Cat5e because that's what was in the truck even though the work order said crappy Category 3 phone cable would do, or even the neighbours are all using the most expensive package and that causes electrical interference that reduces everybody's bandwidth.
You can measure your actual characteristics, and the ISP will offer to do a half decent estimate of that, that's useful to at least decide which service to buy but the average is essentially worthless.
If the UK had deployed fit-for-purpose tech like Fibre-To-The-Home (FTTH), there would be much less need for the UK's advertising regulations. (Obviously speeds are never guaranteed on residential connections, but advertised fibre speeds are a much more accurate predictor of internet experience than DSL or cable.)
"But", you might say, "that BT page is advertising 'Superfast Fibre'!". Well, yes, in the UK it is legal for telcos to lie to their customers. When they say fibre, they mean Fibre-To-The-Cabinet - and VDSL the rest of the way. Would anyone call their 3G mobile service "Superfast Fibre" because the tower is fibre-fed? No. For whatever reason, though, this is totally okay in the UK.