The finale was filmed in three parts, first the scenes with Lisa Kudrow were filmed in Los Angeles, then everybody went to London and they filmed the scenes on the street, then they filmed the dinner and other scenes in a studio in London before an audience.
Lisa Kudrow was pregnant in real life during the filming of this episode, and was therefore not able to travel to London; therefore the writers had to find a way to keep her involved in the story while still in Monica's apartment.
The London street vendor is played by Sir Richard Branson, the founder of Virgin Group. One of the hats in his souvenir booth has the Virgin Atlantic logo on it. Actually there are several Virgin Atlantic baseball caps on the stand; they have tan cap bills.
The trivia item below may give away important plot points.
Elliott Gould (Jack Geller, Ross and Monica's father), committed a major error by inadvertently revealing to the public that Rachel was going to turn up at the wedding, a mishap that upset Marta Kauffman, the show's executive producer.