In reply to wannabeagoat:
Pads go outside the bivi bag. You stop your pad from blowing away by lying on it. Condensation is the enemy and so you want to avoid lying directly on an impermeable thing and putting your warmish bivi bag directly on the cold ground - the vapour will condense at soon as it touches the fabric on the ground and pool under your mat, but inevitably soak into everything else. Plus directly lying on the mat will add its own moisture to the puddle.
Breathable is a relative term.
Exhaled air goes outside the bag too, not inside.
All goretex is the way to go. It doesn't matter if it's khaki green - it's going to be dark anyway.
If it's raining, roll over so that the opening and breathing hole is away from the rain.
Combine that with a synthetic sleeping bag and you're sorted for most things that the UK weather can throw at you. If you have a down bag (ill-advised), then any running water is difficult. If its getting too cold, don't wear bulky clothes, use them as blankets inside your sleeping bag instead.
Don't sacrifice a comfortable night for a few hundred grammes of decent material and a solid bag. If you do, you'll have hours every night to contemplate your folly.
Post edited at 14:56