This is all a bit Daily Mail, isn't it? In my experience, I've had to opt out of filtering each time I got a new mobile SIM with data, and haven't had a problem connecting to VPNs after that. (With T-Mobile and 3, at least, all it took was a short phone call or having a word with someone in the shop.)
I'm sure that the proposed broadband filters, if they ever get implemented, will work along similar lines to this setup. But I think it's alarmist to suggest that that the 'adult content block' - which has been a feature of mobile networks for years - means Cameron's Great UK Censorwall is suddenly springing to life.
Er, I mean... wake up, sheeple! Orwell was right when he said they'd come for our VPNs!