I had a look and have found some C roads (Class III) are unpaved maintained  and a very small stretch of the A848 (Class I) in Salen is unpaved for about 100 metres before it's ending at the pier.
This is the only B Road (Class III) that is unpaved in the UK, some sources suggest it goes down to the very end of Lismore but OS maps show this B Road ending at the gate.
The last 100 metres of the A848 in Salen has been classified as primary on OSM so I would assume this would be allowed too 