You can't really go wrong with an army bivvy, either the UK or US ones. these are a good price (but it is creeping up each week so be quick) ..
http://www.amazon.co.uk/dp/B00B5AE1...de=asn&creative=22242&creativeASIN=B00B5AE1B8
If you want a bit more luxury then look at a hooped version, lots to choose from but they start to get expensive. I have a Terra Nova Saturn which has two loops, plenty of room for kit and enough wriggle room to get undressed etc, spendy though
Best bivy? Carinthia Observer Plus... but won't tell the price 'cos you and you'll have a heart attack.
Try looking for the Dutch Army Gore-tex hooped bivy in camo
http://fu-kit.com/product.php/270/dutch-army-camo-dpm-gore-tex-hooped-bivi-bag-with-pole
If you're really lucky, you might find the double-hooped Dutch one for less than 200 GBP.