Cycling is increasingly niche with destination, distance, speed, the social aspect and terrain all playing a part. Here are some links to get you started. Don't forget that you don't need to join just one!
Club Finder
British Cycling has a Cycling Club Finder page which provides the full details in one listing.
https://www.britishcycling.org.uk/
Kenton Road Club is another option and their rides typically depart from Kenton Library.
Group Finder
The Let's Ride platform supported by British Cycling has a group finder and allows you to join many groups so join Let's Ride and search for groups in your area.
Sample Options
The Harrow Cycle Hub (group) is on the Let's Ride platform and the rides tend to be shorter and 11-18km/h.
The Harrow Cycle Club (group) is on the Let's Ride platform and the rides tend to be longer and 16-20km/h
West London Cycling will offer rides with different speeds.
Gregarios is a club based in Hatch End.
Uxbridge Loiterers are very social with plenty of coffee and pub stops.
Spokes offer rides on the Let's Ride platform and overseas trips.
Willesden Cycling Club typically start their rides outside of London.