It seems in films and TV shows they often put fake plates on the main cars. Here I'm specifically talking about cars in the UK. It seems like the fake plate practice happens more often with newer films. Sometimes they take a registration from another car and attach it the car (or cars) used for filming.

Example:

A Fish Called Wanda (1988): http://www.imcdb.org/vehicle_2277-Jaguar-XJ12-Series-III-1985.html

Shaun of the Dead (2004): http://img262.imageshack.us/my.php?image=shaunjaguar6sy6.jpg

Both Jaguars carry C-reg plates, but one in Shaun of the Dead doesn't look like a 1980s plate, the font is slightly different and for one thing it looks far too new for a 20-year-old plate (made of plastic they get cracks and generally age a bit). But that registration is actually registered to a real Jaguar (but it may be a totally different one). The plate on the Jag in A Fish Called Wanda is actually from a Range Rover.

Can anyone with knowledge of film production explain the exact reasons they do this?

I wonder if they do it so that if it's someone's personal car and the movie becomes a hit, then that person doesn't have to worry about it being stolen or vandalized. There are some weird people in the world that I could see doing that. Similar to how alot of people that post photos of their cars in chat forums usually blur out the plate numbers.

The thing is, ordinary people can't find out where someone lives just by getting their plate number. Only the police are able to do that, right? And even if the film does become a hit, the only movie cars that are really recognisable are the real "star cars" like the James Bond cars and stuff like that (especially ones that carry personalised plates, ordinary plate combinations aren't easy to remember).

I mean how many Sopranos fans know the plate number of Tony's Escalade? Not many at all I'd imagine, it's not information you would carry around in your head to check every white Escalade ESV! I only know because I've spent ages making Sopranos captures for this site!

The majority of the plates, you can see in movies, are faked. As a licence-plate-collector I'm looking for that detail.
Of nicest (resp shabbiest) faked plates I usually make a link to my license-plate-collector-forum. We have a lot of fun with them.

Sometimes the quality of movie-plates is so worst, that it hurts, when you are watching it.

It is even worse when it is in movies with big budget, and where the car has a big role, e.g. http://www.imcdb.org/vehicle_1869-BMW-750iL-E38-1997.html

Have you seen Death Wish V, which has 1973-87 New York State plates on a 1993 Jeep Grand Cherokee?
http://www.imcdb.org/vehicle_55719-Jeep-Grand-Cherokee-ZJ-1993.html


Take a tip from me; if you see a movie with an ambulance that has New York State plates, and they don't include the word "Ambulance" on them, it's 100% fake.