Thanks again for everyone's replies.
The HH ones from Screwfix are certainly interesting, who'd have thunk of looking there! Very good prices and some seem to be lighter weight, though difficult to tell as specs of some include weight and some don't.
I'll sign up for Aldi as suggested, there's a big new one recently opened less than 2 miles from home but I'll take my digital scales if I go there! (Also a Lidl opening soon on the route there, but baselayers I saw on their website were 43% cotton, say no more.)
Patagonia Capilene Lightweight excited me most from the weight point of view, but they're at the upper end of what I would pay, while they appear to be in short supply, I think I'd find it difficult getting both top and bottom in my size from the same supplier.
As for washing them as I go, my trips are 4 days max, and the sheep don't complain.