It really depends on what you want to support.
At national level, you want to support the CTC.
If you're particularly interested in London, you want to support the LCC. (Disclaimer: I'm a trustee, so please see this recommendation in this light.)
@Oliver Schick started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
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It really depends on what you want to support.
At national level, you want to support the CTC.
If you're particularly interested in London, you want to support the LCC. (Disclaimer: I'm a trustee, so please see this recommendation in this light.)