The UK has to do that because the fastest widely-deployed internet connections available are VDSL2. DSL connections in general are relatively unreliable, and speeds vary wildly depending on a variety of factors, most importantly the length of the cable between you and your cabinet/exchange. Australian folks may chime in here with tales of their DSL connections dropping out during heavy rain, when their telephone junction boxes fill with water and short out.
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.
On the flip side, in the early 2000s Bellsouth (now AT&T) deployed FTTP in much of the Southeast and would convert that to ADSL to plug into existing house POTS wiring. It was sold as ADSL at 12mb/s. There was literally no other internet option available from them at the time either.
The averages are basically pointless. They might as well publish average average book lengths for categories on Amazon. Want a young adult novel? Average price 4¢ per page! Oh you want a specific novel? Sorry we advertise averages only.
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.
And it's not like that everywhere, the UK has started what I hope develops into a trend: https://www.bt.com/broadband/deals/