If you are in London now, keep an eye out for the Tweed Run cyclists tomorrow (Saturday). This is a group ride of 750 -1000 cycling enthusiasts, ideally on vintage bikes and wearing vintage (Tweed) clothing. Here is a link to their etiquette page; you can navigate to photos of past rides from there.
http://www.tweedrun.com/etiquette/
We saw them in May 2014 riding on Fleet Street heading east. Later in the day we saw them close to Westminster Abbey. They do not publish their route to the public ahead of time, so you may not be able to find out where you can see them. In May 2016 we missed them entirely. But it is well worth looking for them if you are out and about tomorrow. Lots of fun and good cheer.