Hello Saviola
The 20K service interval is an option, designed for drivers doing mostly long trips, usually on motorways and appeals to fleet managers. I do about 20K miles a year, so my car is set to this "long life" interval, where the oil quality is monitored to decide when a service is required. The maximum interval is 2 years, or about 19K miles, whichever comes first. I would think if you are commuting into London, with its stop/start traffic, the fixed service regime of annually / 10K miles would be more appropriate.
Good look with your car hunting!
Glen