I'm not british, but I feel hard to believe that more people are attending boxing than RU, RL or Tennis in the UK.
I believe for total attendance numbers, these guys are a lot closer to the true (RL is ranked 4th there behind soccer, cricket and RU, and also horse racing, but who cares)
https://twocircles.com/gb/articles/2024-sports-attendance-review-uk-edition/
Same with Australia where RL is ranked 2nd
https://twocircles.com/gb/articles/turnstiles-spin-as-sports-fans-pack-australia-venues-with-record-growth/