Jake,
In London, they use Miles, not Kilometers. (the "k" Gerald used in his post means thousand, not km).
Gerald123 and everyone else,
Whenever I go to Europe, I lease (not rent) a car and I always thumb through the owners manual. The last two cars I leased said to change the oil and filter "every 30,000 km". Getting out my handy dandy calculator, I calculated that 30,000 km is 18,750 miles. This is with gasoline (petrol) engines. Don't know about Diesel engines.
Note that European Synthetic Oil (Group IV and/or Group V) is better and longer lasting than American Synthetics (Probably Group III) or even non synthetics.
I remember years ago, when Mobil-1 was the way to go in Synthetics, they advertised it would go 25,000 miles. I also remember that I worked for a Chemical Engineer who firmly believed in Mobil-1. He had a Honda Civic that he would changed the oil once a year, in the springtime. He normally put on about 22k miles a year. He would change the filter every 8k miles, and top off the engine oil to replace what he lost from the filter change. I know he did this for the entire four years I answered to him, and he never had any problems with that Civic. (Amsoil [at one time called AmzOil] also advertised 25k mile oil changes, but back then, like today, it was so much easier to buy Mobil-1 than to search out and buy Amsoil).