I'd hazard a guess, but I'd say that the vast majority of bikes bought from Halfrods et al, end up at the back of garages with no more than 20 or 30 miles under their belts?
Fact is, if you do your due diligence, you could end up with the bargain of the century. If you don't, you could end up buying a nice looking frame needing £500 worth of parts to make it properly rideable.
Not just the cheaper
Halfords bikes either. My '88/89 Raleigh Gemini hybrid was a fairly expensive bike in it's day, being 531 and having some decent quality mechanicals. That was supplied new by FW
Evans in Kingston, according to the dealer sticker. When I bought it in 2018 for £20 it still had the original Michelin World Tour tyres, and they weren't worn just shabbly. It had only had either one or two puncture repairs (can't remember which), and mechanically it rode like new - and still does. At a pinch, it might have done a couple of hundred miles, which is peanuts in 30 years.
Much the same story with my 1985 Raleigh Royal tourer. Expensive bike new, paid £30 for it in 2019. Original Michelin tyres again. One repair patch in a tube. Mechanically very smooth and quiet, not the bag of spanners feel you get from knackered high mieage machinery. few more minor dings and scrapes where it had been leaned against poles etc, but I still reckon the mileage was in the few hundreds, no more and that was spread over 34 years! In both cases we are probably looking at a yearly average of no more than 10 or 20 miles having been ridden.