For some hobbies and businesses, it's necessary to use Coordinated Universal Time or UTC, which is similar to Greenwich Mean Time (GMT). The iPhone World Clock can display several separate clocks, so I thought it might work to add London as a substitute for Greenwich, which was not listed. But on a lark I typed utc in the search for city dialog, and it came up! Once selected, I had a clock dedicated to UTC. This is better than using a nearby city, as it should avoid problems with daylight saving time changes.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080402223345694