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.
Companies have legal departments, which exist to figure out answers to questions like that. This is because these questions are extremely tricky and the answers might even change as case law trickles in or rules get revised.
Expecting individuals to interpret complex rulesets under threat of legal liability is a very good way to make sure these people stop what they are doing.
The law worked the same way yesterday as it does today. It's not like the website run in Britain operated under some state of anarchy and in a few months it doesn't. There's already laws a site has to comply with and the risk that someone sues you, but if you were okay with running a site for 20 years adding a report button isn't drastically going to change the nature of your business.