I can zoom in to a scale of
40 inches to the mile using Bing maps in OS mode on my laptop for free, but that's not the point. The problem is that the laptop screen isn't big enough for planning a 20 mile walk, let alone a 60-70 mile ride on a phone screen, so if I zoom out enough to see a useful distance on the screen, all the detail becomes unreadable. A close up view is fine if you just want to see as far as the next junction, but not for trying to visualise a complete route between start and destination whilst you're planning it.
The width of a Landranger map is only 24 miles, you can cycle two, three or four times that in a day, it takes 28 sheets to cover my most recent tour: enough to almost fill one pannier. To plan a 60 mile ride with a road atlas I can lay a couple of pages on the table, then when I get on the bike, a folded page in the mapholder will give me enough coverage to keep me going for a couple of hours or more before stopping to turn it over. With an OS map on a mobile phone zoomed close enough to make it readable, you can cycle across the width of the screen in a few minutes.
On the left, a page from my 4.2m to the inch atlas folded to fit into my mapholder (made from a CD case) which covers an area of 17x22 miles. All the roads, right down to the minor ones, are clearly visible, unlike the same area covered by a Landranger map shown on the right:
View attachment 775719
My road atlas, covering the entire country, occupies about the same pannier space as two Landranger sheets when it's folded up in a poly bag.