If you are going to Larkhall the route from Blackwood through Netherburn and Ashgill is much nicer and not much longer, there's crap pavement cycling between Larkhall nearly to Hamilton, but the cycle route gets very recreational and meandering after that.
If you are going to Strathaven the route that goes through Waterside (~NS785371) from Lesmahagow to join the B road at Kypeside is nice, but might be a bit of a diversion and whilst it would miss out the crap bit of road before Tescos at J10 you won't be able to indulge in their culinary delights either.
You could do a loop out from from Abingdon or Happendon Services through Biggar, Peebles, and Innerleithen and down through Eskdalemuir back to Gretna, it would probably be somewhat more scenic but rather less Glaswegian.