While there are some specialty machines that are capable of relatively high velocity, your average run-of-the-mill helicopter can't sustain speeds much higher than 100 miles per hour. That said, because of the heli's ability to climb high into the air with cameramen packing extreme telephoto lenses, actually managing to trick a chopper pilot into losing you – in scenes sure to be played out monthly on national news channels – is another matter entirely.
The Isle of Man is a veritable gearhead's paradise. It's located in the Irish Sea right between England and Ireland, but is its own independent island. Like Monaco (and a few other examples around the world), the Isle of Man has no speed limits. Which could explain, in part at least, why Jeremy Clarkson and Nigel Mansell chose to live there.