Geograph, of course. I didn't think of that. Thanks.
I'll give it a go. What could possibly go wrong? .... BOOM!
I did almost the entire length of the Welsh/English borders with my pony in 2007 with
very few issues, largely by using Geograph combined with OS maps, and local knowledge wherever available. Met some wonderful people, too!
I have had too many horse-journeys which have almost ended in disaster, and which have certainly had to be much re-routed, because of condition/inaccessibility/unusability of an essential bridleway - even when there were precisely zero problem reports about it on a county's definitive map. Correct information when travelling by equine-powered transport is vital. You can't take a horse to pieces and put it into a bus or taxi, or lift it over a barbed wire fence or locked gate which blocks your way. Nor can you take its front legs off to get it through a narrow barrier bit by bit ...
Crossing or travelling along, a dual carriageway or similar is even
more dangerous and unpleasant on a horse than it is on a bike. The bridleway network is fragmented to say the least and 'lobby groups' such as Sustrans have done a great deal of damage to it, while providing almost no further usable (to horses) mileage of bridleways. Their so called multi-use tracks seem, in the main, to be specifically designed to be off-putting or even inaccessible to horses - even when said track is actually based on and around a bridleway route. I have come across some simply ridiculous things on sustrans routes - routes which had been perfectly acceptable bridleways before that lot took them over and built barriers that horses couldn't get through, 'improved' bridges so that horses couldn't/wouldn't cross them and surfaced tracks with slippery, lethal, totally inappropriate surfaces ... fortunately that was a shortlived phase but the legacy lives on and so-called multi-use paths are vanishingly-rarely so in practice.