Something like a Cheviot similar to P&H Hartwist? I would have a hard time imagining that W. Bill wouldn't have one. Did you stop by their basement to view possibilities? Being in London, you have no excuse not to! :^)
Wait. Scratch that. I thought this was the tweed thread for a second. If suiting, I would think this one below should be nice from Dugdale Royal Classics.
Saw a nice navy herringbone in the Lesser 13oz book, probably slightly lighter than true navy which I think is actually preferable. However, I vaguely recall GDL had one made up in that fabric and wasnt a fan...
No I like it a lot actually. In hindsight, the shade darker would have been more CBD and versatile but the cloth is great. It's bullet proof