I contacted the UK Government recently online about tax issues and they asked me questions to confirm my identity .
They asked me how many bank accounts I had , I got that question right , they asked me when I took a UK phone contract out , I got that question right , they asked me when my Passport expires and I got that question right .
Asked a few more questions and confirmed that I was me
I hadn't given that information to anyone, yet the Government still knew the answers .