Interesting thread, and nice to see that some people still actually navigate rather than simply get told by a computer chip where to go (pun intended).
I personally think that Explorer (1:25000) maps are necessary for navigation in open areas (my local one is Dartmoor for example). A while back I walked the Cambrian Way, and found the Landranger (1:50000) fine for the earlier, more built up areas perfectly functional, but once on the Brecon Beacons the Explorer was king. It's also worth mentioning that since OS is a public service, they are very good at listening to their customers (i.e us). It is therefore worth informing them of any issues. On the Cambrian Way walk there was one point (near the source of the Severn) where there was a gap of about a mile between two of the Explorer maps that didn't appear on either of them. I bought the 'missing' map because you never know what might be there, and sure enough, there wasn't anything to worry about. I wrote to OS who replied, promising to look at it for future publications despite pointing out that the Cambrian Way isn't an official National Trail.
With regard to foreign mapping, aren't we lucky to have the OS? OK French maps may be messy but they are usually accurate. Spanish? I still have two overlapping maps bought at the same time that have a named place in two different locations. Greek? How does anyone get anywhere? I also have a map of the Mount Elgon National Park (border of Uganda and Kenya) that shows nothing (including grid lines) but contour lines. That really does challenge the navigator, even with a GPS since there's no map datum info either.