Yes, chary is right - the Berghaus (UK) website brings up 2 waterproof jackets for women up to a UK 20.
And North Face - 4 in XXL, quite a few in XL - although their website isn't especially clear in the sense that it gives measurements, rather than sizes (18, 20, etc), as the numbers for what constitutes XL or XXL. (They don't seem to show a chart for the XXL at all.) The measurements are probably more helpful, but I don't have any idea what they correspond to in terms of 'standard' sizing.
(However, Gore-Tex itself is the actual trademarked waterproof/breathable fabric, from which lines such as Berghaus, North Face, etc garments are constructed.)
I had an impossible time finding waterproof trousers over a UK 18 here - if anyone can spot them on t'internet, let me know!