This recently appeared on a (not 'the') london critical mass website:

Jenny Jones has sent the following letter to the Commissioner of the Met Police, regarding Critical Mass. She and Darren Johnson will attend the next ride to show solidarity with fellow cyclists. They hope that others will do the same.

Sir Ian Blair
Metropolitan Police Commissioner
New Scotland Yard
Tower Block
N5Y

Dear Ian,
Critical Mass Bike Ride

I am writing to outline my concerns about plans to label the Critical
Mass Bike Ride as a demonstration and therefore require it to have
permission from the police as well as permission to travel through the
restriction zone around Parliament.

1) Many people do not see Critical Mass as a demonstration, but more
like a hundred people getting on the same train at London Bridge.
2) According to the public order branch of the MPS, Critical Mass
would not meet the definition of demonstration set out by the Serious
Organised Crime and Police Act 2005. The bill requires that the protest is
static and has a political message.
3) Critical Mass has operated peacefully in this country for 11 years.
There is nothing to suggest that this is likely to change and the
heavy-handed application of the Serious Organised Crime Act and other
legislation is surely unnecessary.
4) Given the fact that Critical Mass has been happening for so long it
may be the case that it does not need permission under the Serious
Organised Crime Act.
5) Operational issues may arise in attempting to arrest attendees at
Critical Mass. How will the police distinguish between ordinary
commuters who are going about their business and those who are attending
Critical Mass?
6) I shall be attending the next Critical Mass as a direct response to
attempts by police to ‘crack down’ on it. I can only imagine that
dozens of other Londoners will feel similarly obliged to stand up for their
right to cycle around London.

I urge you to review this situation and allow Critical Mass to go ahead
unimpeded.

Jenny Jones
Green Party Member of the London Assembly
Mayor’s Road Safety Ambassador
Member of the Metropolitan Police Authority