It's got cold on the mean streets of London. Cycling is more challenging. The best item I think I've ever bought for the cold (apart from the obvious - gloves, something to go on your legs, waterproof jacket) - is a neck cagoule from Mountain Equipment Co-Op in Canada. It's cheap but it just keeps your neck brilliantly cosy.
Just spotted that they're sometimes called neck gaitors, or neck warmers. This is the kind of thing I mean: