In the UK it has never been compulsory to carry ID, even when driving.
At a traffic stop the police have the option to require you to present documents at a police station within seven days if they think something is fishy.
And people do seem to exist quite happily without formal identification. As someone who has always had a passport and driving license it was a bit of an eye-opener to me, but if you don't drive and don't travel, some folks just get by without.
So if there is a requirement to have a Britcard, and to present your 'Britcard' when stopped for any reason, then it is definitely a change.