Oh, i was a little suprised at how low people rate the foil blankets. I havent used one before but do tend to have one with me on winter rides.
The thinking is that a foil blanket doesn't help in typical hill (esp. UK) conditions, for 2 reasons:
1. They are good at preventing heat loss due to radiation. This is fine in somewhere like, say, the Alps in winter on a sunny day where the air is clear, still and very cold. However in the UK, typically you're going to lose heat through convection, not radiation, because it's wet and windy. Foil blankets don't help, the old-school orange survival bag is much better because it keeps the wind and rain off.
2. If you try to use one in the hills, it WILL get utterly shredded, from your own efforts as well as the wind. Seriously, has anyone even unwrapped one indoors? They're hopeless.
The survival bag with foil blanket taped-on that Doug and I refer to is the recommended method on all the outdoor leadership courses (hill walking, climbing, etc. as well as biking) because it solves both of the problems above.
All that said, I carry a group shelter most of the time when I'm guiding, which is far better than either option above!