Planet Patrol (formerly Plastic Patrol) runs a free app for logging litter and water quality, organises paddle and walk clean-ups, and publishes brand and location analyses with university partners.
Honest caveat: it is a Community Interest Company (11417810), so there is no Gift Aid; its recent work includes airline-sponsored clean-ups, which some volunteers may want to know.
How to start: planetpatrol.co/download-the-app/