You will always start at your gateway - so if you had to fly from Northolt with LHR as your gateway, the company will provide a car to transport you there from LHR. Same thing if you live out to the west, say Reading, and had to start at Farnborough. You could negotiate getting yourself there and not need to use a company arranged car. I also know of some personnel who arrange hire cars through the company but am not too sure how that works.