@[b]Superexcited[/b]: I think I remember being able to put a manual car into a higher gear, once it got going (e.g., 2nd to 5th instead of 3rd), during driving lessons, and being told that my mistake was technically fuel efficient. Perhaps manufacturers are taking that into account? You're also taught to put the car in neutral and use the handbrake when stopping for any length of time with a manual. In an automatic car, you're more likely to leave the car in "D" (in gear) and keep your foot on the brake pedal until it's time to go again.
As far as real-life costs go, I think it kind of evens out. If you're buying a car new, or barely used, I imagine an extra £1,000 on a ~£20,000 loan would make a difference, but maybe not that much of a difference, on monthly payments. Not getting out a spreadsheet to check, though. :-p We bought my baby used, with cash (that's how used...), so that part wasn't an issue for us.
In addition, it's only anecdotal, but my husband's beloved manual Golf is a diesel. He is technically supposed to get even better mileage with that type of fuel, but the cost per litre of diesel is so high, and he does so much driving around the city, that we're spending about the same per month to fill up (my Rav being a petrol car).
I do think automatic engines in Britain are getting better every year. If my experience driving a Micra in Ireland during 2006 is any indication, they used to be rubbish - no power and ate fuel. The car I have today drives pretty much like any car I drove back in the US.
Weather conditions can be challening, but most decent automatic boxes have some lower gears that you can force the car into. Handy for snow.