As far as I know LA in the serial number indeed means LA cycles. They made some of the fillet-brazed XTCs for Thorn, my partner has one and the frame is excellent. If you look closely in the photos this Brevet seems to be fillet-brazed too, so this really looks like a UK-made frame, and a nice one at that. Reynolds 525 is maybe not the highest-end tubing but Dave Yates seems to be using it for his tourers, so it can't be bad I guess. A really nice bike overall.