The instructions to get to the Metropolitan Police Car Compound from the Underground were a little vague on the police website, so I have included walking instructions below. I takes approx. 20 minutes.
Come out of Perivale Station and turn right under the railway bridge, after bridge turn right into car park and walk all the way to the end (thru) car park and turn left.
Walk down the left hand side of the road past the garages. You will see a building in front of you with a sign saying Danly, bear left onto Aintree Road.
Walk for about 1/2 mile until you come to a road called Walmgate Road, turn left up there and you will see the police compound right at the end.