I stand corrected. The Road Traffic Act 1988 Sect 192 defines a road as:
(a) in relation to England and Wales, means any highway and any other road to which the public has access, and includes bridges over which a road passes, and
(b) in relation to Scotland, means any road within the meaning of the Roads (Scotland) Act 1984 and any other way to which the public has access, and includes bridges over which a road passes
The only exception would appear to be if it is marked Private and access to the public is barred.
Therefore, the national speed limit would apply to farm/forest tracks even it such speeds are impossible.