If you're building a motorway - and we've all done it - there will be occasions on which it is necessary to provide a short spur road to connect your new highway with some part of the existing road network that is slightly out of reach. Because the little spur inevitably leads to a motorway, it should be a motorway too; otherwise cyclists and horse riders might get to the end and find themselves stuck.
There is, however, no need to number the little spur. It's just part of its parent route, like the little M23 spur near Gatwick or the branch from the M6 to Wigan. It's like a set of extended sliproads.
Every rule has exceptions, of course - especially where motorways are concerned - and the M181 is one of them. This piffling little connection from the M180 to the A18 west of Scunthorpe has ideas above its lowly station and, for some reason now concealed by the ragged and threadbare dungarees of history, was granted its own number when other spurs of greater length do perfectly well without.
This lengthy discussion of the road's spurious number (pun very much intended) is necessary because there is nothing about its physical manifestation that is worth discussion. It has a fairly smart trumpet interchange with its parent, the M180, and then proceeds in a straight line through perfectly flat countryside for a couple of miles before giving up at a roundabout on the A18. It passes nothing of interest, and its surroundings are so featureless that Scunthorpe is the closest settlement to both ends. There really is nothing more to say.