Basically to stop people from using 10year old outdated browsers with huge security holes in them.
The change from an opt-in update to an opt-out update happened in January in some parts of the world, it's obviously been implemented in the UK this week.
although you may see it as an inconvenience you are far far better and safer running an up-to-date browser, earlier versions of IE were insecure and they don't support a lot of the emerging technologies that are starting to be used across the internet so you'd also find a lot of sites simply would not work with older browsers.