They don't have a national ID system though. Having a lot of different ID systems that are IDing other things for other purposes doesn't address this.
> My tech mind tells me that it's just going to save some DB admins from having to JOIN some columns.
I don't know how this works in the UK but I do know a bit about how this works in other jurisdictions. The data in the current systems are there because there is a law that says what data is collected and for what purpose. You can't lawfully use it for a different purpose (there might be a loophole for public safety or whatever, but that would be an exception). Your organisation would break the law and the bosses could go to jail. But also, it was designed for one purpose and if you tried to use it for a different one, you would run into a lot of data quality issues.