Reccomendations for off the peg courier bags?

pip Off the peg is hard in London. Chromes are pretty solid (Condor carries these) and I think Brick Lane has a small stock of Bagjacks. I'd go riding around and trying out options. However, you may find that the prices aren't that much better than custom (Bugaboo, Project Tortoise, Archies etc).

I also personally wouldn't get a one designed for a laptop. Just get a normal messenger bag and a padded sleeve; viola a much more flexible solution.

what do laptop sleeves have to do with big violins