This needs to be seen in the context of the political situation in the UK. Boris Johnson, the current PM, is currently on this 9th and last political live. During the strictest period of lockdown, when UK citizens where not able to say goodbye to loved ones dying in hospitals, several parties were organised at his residence . It is alleged he attended two of them himself and was photographed drinking beer. In attempt to rescue his political career, he has announced several "red meat" measures and this appears to be one of them.
Over the years several attempts have been made to introduce this. Just like past attempts it will face a heavy lobby from the adult content industry and social media companies. More importantly this government will likely not be around long enough to actually introduce it (the consensus is that Boris will be gone after the May local elections). It will almost certainly drop off the radar of a subsequent government as there are much bigger and more important issues to tackle after Covid.
In the unlikely event it will get introduced, it will be delayed for years. For past measures Ofcom, the media regulator, was asked to classify all adult websites to determine if they fall in a category that requires age verification, and as you can imagine, this can take a while. In addition age verification is only required for websites that allow users to upload content, not commercially sourced content. There are endless loopholes here, however, likely adult websites will just block user content from UK visitors. It is not like there is a shortage of commercial content. Of course VPN providers will do well out of this too.