Not related to the supercars you're always seeing, those aren't a big issue in January.
Car theft is a major problem in London, as it is in any number of major metropolitan areas. But police in west London, Kensington and Chelsea in particular, have seen a major increase in thefts of expensive cars as theft rings now posses the technology to clone electronic ignition keys, easily sidestepping manufacturer security measures. The equipment needed for this isn't particularly expensive either, so know-how is all that's required.
It is BMWs that are particularly vulnerable to this, but Range Rovers are a not-too-distant second. Of course, any cars which use these types of ignition systems are theoretically vulnerable, and the disproportionately large numbers of vehicles from these two manufacturers might simply be to do with their popularity. So police in these areas are now randomly stopping expensive cars seen out driving after midnight, when most thefts occur. Naturally, police can't expect to catch everyone, and have issued a statement reminding car owners that steering wheel lock keys are not so easily cloned.