As DanW said, use Streetview or Google satellite views.
I also have Viewranger open which shows NCN routes on OS maps.
It is also worth saying that Bing maps can display OS maps so when planning an offroad route I’ll use a combination of the OS map, Street View and Satellite views to get a feel for is a road looks reasonable or offroad section rideable, then plot on Garmin Connect.
Worth noting also that minor roads can be local rat runs or very narrow and twisty so dangerous in other ways to main roads so asking locals for a bit of advice can be extremely helpful (have had some great help with route planning here on STW).
Also, with all this in mind, I don’t think any software can account for all of these factors in planning a route for you. Without a bit of online recon and advice from locals it is highly unlikely to ride 125 miles without hitting a dodgy road or two. Generally, I build up my network of known decent routes and gradually expand in different directions and in placed that look good on Streetview/ OS map so at worst only a small portion of the ride is likely to be iffy. You can also explore sections of decent road on Strava segements and browse routes on Garmin Connect to get some good inspiration. Still worth Street Viewing the bits you don’t know to not follow a nutter blindly on to a dodgy section of road!