A cycling site with 275k MAU would be in the very lowest category where compliance is things like 'having a content moderation function to review and assess suspected illegal content'. So having a report button.
Im surprised they don’t already have some form of report/flag button.
That means you need to do CSAM scanning if you accept images, CSAM URL scanning if you accept links, and there’s a lot more than that to parse here.
• A "large service" (more than 7 million monthly active UK users) that is at a medium or high risk of image-based CSAM, or
• A service that is at a high risk of image-based CSAM and either has more than 700000 monthly active UK users or is a file-storage and file-sharing service.