The problem is, Ken, that a lot of the required vehicles, especially for dramas set in earlier times, are simply not available. I have already mentioned the pre-1945 London bus problem.
Heathfield's gripe with railway liveries is particularly relevant. But once again, despite the vast number of heritage railways operating across the country, to get a locomotive, carriage set (not to mention a station or two) which precisely maps the area and the era is extremely difficult. Many heritage railways use post-nationalisation rolling stock (it's more robust, more plentiful and easier to maintain under heavy usage). All in all it's very difficult. I work as a volunteer on one of the country's foremost heritage railways which is used for a lot of filming work. The producers do all they can to get their scenes as authentic as they can, but they usually have to sacrifice some authenticity for the sake of practicality and, of course, cost. As an example, despite being set in Yorkshire, railway scenes for the lavish (and presumably high budget) Downton Abbey are filmed on the Bluebell Railway in East Sussex. "Downton" station is in fact the railway's Horsted Keynes station. Everything there is painted in Southern Railway green and cream and is mainly representative of the railway's appearance in the 1930s and 40s. They usually use a locomotive more representative of the Downton period though not one which would have been seen in Yorkshire. Carriages used in scenes in the last series were actually the railway's "Chesham" set which, before saving for preservation, actually ran on the Metropolitan Line of London's Undergound!!
It would nearly always possible to obtain the authentic motive power and rolling stock as there is plenty available across the country representing nearly all geographical areas and eras. But the cost of doing so for a day or two of filming is prohibitive. Most producers and location managers concentrate on what they think are the important aspects of their scenes because they know that there are not too many "enthusiasts" like us who will pick holes in their productions !!!