I share you're disappointment at not being able to access the Christian Concern website from my O2 mobile.
Well, of course you do, you followed a link to get here..
But you need to contact customer services and make the request. And tbh, I'd be asking the "some venues" why, as it has been noted that the individual premises can block sites without o2 input. Coming on to a site and making a clever username of Holmes backwards doesn't make you a detective if Barking up the wrong tree.