In the UK you may be able to get Rasant thread. Here in the US, Mettler Metrosene Plus is the best I've been able to find in a variety of colors.
Some of the low denier, high DWR materials would make a good inner tent or liner - like the Momentum at Thru-Hiker, available in many colors, if you don't mind the shiny calendered surface on the inside. Don't know what is available from Extrem. I have some 'showerproof' 1.3 oz/sq/yd polyester from the UK that won't sag, so you ought to check out all the materials sites in the UK first. You could go lighter in the US than one oz., with good DWR, but for more money, and if you can find a color you can live with, and get it shipped to the UK.
For the outer, I guess it would be .18 mylar cuben at around one oz/sq/yd also; but again it's a question of whether you can find and obtain a shade you can live with. If not, then there is the silnylon that Stuart got from Extrem and sounds AOK with, or T-H's silnylon in either gray or yellow.