I don't like the OSA and associated regulations as much as the next person -- I think we could have gotten a long way by saying you need to include a X-Age-Rating in http responses and calling it a day. The law itself is incoherently long and it's very difficult to know what duties you have.

However, I don't see what the legal basis of Wikimedia's challenge is. The OSA is primary legislation, so can't be challenged except under the HRA, which I don't really see working. The regulations are secondary regulation and are more open to challenge, but it's not clear what the basis of the challenge is. Are they saying the regulations are outside the scope of the statutory authority (doubtful)? You can't really challenge law or regulation in the UK on the basis of "I don't like it".