ITMA wrote:
What are these cars with the siren, and annoyingly dazzling dangerous
alternating-flashing-headlights as have become trendy on real police cars,
that sometimes barge their way through London traffic?

Just ordinary, low powered everyday small family cars, not unmarked police
cars in the conventional sense at all.

They've come up from nowhere in the last year or two, particularly prevalent
in the last couple of months.

What makes me wonder if they're genuine is

(i) there's been no formal announcement that they are kosher, and

(ii) on one occasion I saw the same car on three consecutive days at the
same time of day in morning rush hour traffic going into London approaching
the Hammersmith Bridge in the bus lane along that road, using its flashing
lights to get to the front of the queue then switching them off.

They're Flying Squad "gunships". For some reason one of the Sweeney's
main units is based in leafy Barnes so seeing them in Hammersmith is to
be expected.