The alpha version of GOV.UK was using MySQL and PostgreSQL. GOV.UK beta is based on Amazon RDS (MySQL) and MongoDB. In there words:
We started out building everything using MySQL but moved to MongoDB as we realised how much of our content fitted its document-centric approach. Over time we’ve been more and more impressed with it and expect to increase our usage of it in the future.
Here’s how GOV.UK architecture looks like:
Original title and link: MongoDB at GOV.UK: The Power of the Document Model ( ©myNoSQL)