No, it's a ridiculous idea. The most obvious reason is that there's no guarantee of the security of the service holding the scans.
There's also the blindingly obvious fact that as soon as they try to implement this, everybody who wants to visit those sites will grab a cheap (or free) VPN service and bypass it entirely, and there's no practical way for any website to determine the location of an individual using a VPN.
Then, of course, the government will try to ban VPNs (they've tried that already, but this time they'll have an added "Won't somebody think of the children?" argument), regardless of the fact that tunnelling is one of the basic services required to keep the Internet running and secure (not that MPs will ever understand that).
In fact, I'd guess that trying to ban VPNs and other tunnelling services is the actual reason for this - they make the security services' job harder because of the encryption involved, and this government really doesn't like its citizens having access to high-grade encryption.
An alternative to requiring websites to implement this is to put the scanning tech at the ISPs - but, as things stand, there's no way that ISPs have the capability of decrypting VPN connections on the fly, so that's defeated too. Unless...the government "helps" them stay legal by putting GCHQ's realtime decryption tech in there (something that's been defeated several times already).
Which, of course, is something the security services have been trying to do for nearly two decades now.
Don't kid yourselves that this is anything to do with porn and protecting children. It's just a way to try to get a decent chunk of the population to support something that's really, really not good for them.