Lots of cars on the screen can be traced on the Internet Movie Cars Database. http://imcdb.org It is a "timewaster" site so allow plenty of time when you visit it!
As another sad TV and film classic car spotter, I too first thought that the car was an MG, but then realised it wasn't. I use an Iphone App called "My Car Check" where you simply enter the registration of any car and the App will tell you the make, model, colour and age of the car. Interestingly for ALU 453 it does correctly identify it as a Alvis Speed 20, but has its colour listed as Green. I could have sworn it was burgundy red.....