This feature is available for UK advertisers and I’ve actually been able to set this up for a UK geo-targeted campaign, although as far as I was aware it’s only US data which is available for demographic bidding on US geo-targeted campaigns, maybe not?
Here’s a screenshot of the demographic campaign I setup (click for full-size image):
There’s also screen captures on Search Engine Roundtable to show the demographic targeting campaign setup for a US geo-targeted campaign.
I’ve only just set this up but I’ll run off a report and update this post in a couple of days to show any data and see if this is actually working in the UK too.