I like the bit where it says you have to lie face down if it rains... Call me fussy, but in the UK, you'd better get used to sleeping on your front. Stepping back though, bivvies are always going to be a trade off in the sense that you give up a certain degree of comfort in exchange for light weight. In the UK the relatively high humidity, relative to cold, high mountain environments, means that breathable fabrics don't work as well and the frequent rain makes the mechanics of using a bivvy bag more complicated - it's stuff like stowing your - probably damp - sleeping bag, dressing and undressing, cooking, packing, keeping your pack dry etc...
I'm not saying it's impossible and those of a less wuss-like persuasion than myself may revel in the challenge, but I've yet to be convinced by UK bivvying on a regular basis.
That said, I'd go for a Therm-a-Rest, a synthetic bag and a bivvy or hooped bivvy with a closeable entrace and some sort of flap that'll let you leave it part open without you getting soaked. Sleeping in a fully sealed bivvy bag is claustrophobic and unpleasant. Bivvying on a clear night, at 6000 metres, with a warm sleeping bag and watching shooting stars streaking across the sky on the other hand is one of those things everyone should do at least once.
Oh, one last thought, I always put the bivvy bag on top of the mat to protect it from dirt and abrasion.
If you do go for a Gore one, the Mountain Range stuff is cheaper than most and perfectly useable.
