And how exactly will they enforce this?
Exactly!
By forcing everyone to identify themselves before logging in. You will either need to upload an image of your ID, or provide a credit card/bank account number that has already identified your age.
By enforcing the law they already have in place. No one in Britain will be allowed to login to any site on the WWW without presenting a photo ID at time of signin along with your face.
That way, when you say anything critical of Brit gov’mt they will be able to simply put you in jail without a trial — since they have already gotten rid of jury trials.
Imagine the consternation here in the US when (according to the DNC) blacks are too stupid to get an ID.