I’m looking for a desktop (web or programme) mapping programme that works like Komoot or Strava Premium (kinda auto-filling the route between points, but uses actual OS maps as the underlay. Most times that i plan a route on komoot or strava for anything other than road riding, i have to export it to memory map to see if it works & goues down the correct trails.

OS maps online has an option to automatically follow paths and roads when plotting a route. Syncs easily to other devices with the app too.
https://explore.osmaps.com/

^^ Mibbe different darn sarf, but up here snap to path only works in national parks.

I think the newer site (link above) offers that more widely (might be wrong though as I’m still using the old one at https://osmaps.ordnancesurvey.co.uk/). I tried the new one a while back when it was very new and it was pretty awful when editing files imported from elsewhere. I haven’t tried the newer site again yet, but I did report the problems I was having so hopefully they have improved.

Try RidewithGPS with the OSM Cycle layer. I generally use the walking mode and manually avoid footpaths as cycle mode sometimes finds sections of bridleway it doesn’t want to use and chucks in a big diversion.

The Strava autorouting/mapping is complete pants in comparison.

As above OSMaps site. Or justgoride.co.uk which also has OS layer and does snapping.

Cheers, will take a look.

As above the new OS map site snaps on the standard map layer now.

Plotaroute.com has an OS layer on the paid version. It also has very versatile route mapping and editing tools.

Garmin Basecamp, with the TalkyToaster maps