GCHQ has metadata on all digital communications - even among homeless and immigrant populations have near 100% mobile daily usage.

"We" surely have pretty good information about number of adults in the UK, and if the security services are worth their salt we know their names and associations.

Heck, the main supermarkets can probably tell you within a percent or two what the live demographics of the country are.

Well, that's what you get for not running a totalitarian country.

By and large, it's a feature, not a bug, that the government isn't sharing all the information it has between its various parts.

Eg GCHQ has lots of information it has (ostensibly) for keeping the country safe, but that doesn't mean that the prosecution in a criminal case should get access to all the same information.

Of course, that's a bit inefficient and duplicates efforts. But such is a price for restrained government.