It is very likely, that you are suffering from a problem at Telia Sonera. They have major network problems in London, close to Canonicals own networks that host Ubuntu related services.
To see if you are affected, open a terminal and make sure traceroute is installed (
sudo apt-get install traceroute), then type
traceroute 126.96.36.199 | grep --color=auto telia. If your universe repository is also based at Canonical (e.g. not a local mirror), see below on how to install traceroute withouth apt.
If this command outputs anything, then Telia is most likely the cause of your problems.
If this command does NOT output anything, it is still possible, that you are affected, because of asynchronous routing.
This problem is not within Canonicals reach, so they cannot really do something about it (besides building global redundancy), but it should be temporary.
(This answer is only valid close to the time of writing! If you read this in a couple of weeks or months, then this does most likely NOT help you!)
How to install traceroute without apt
You can download traceroute from a mirror that is not affected by that issue and install it via dpkg.
For amd64 arch run
wget http://de.archive.ubuntu.com/ubuntu/pool/universe/t/traceroute/traceroute_2.0.18-1_amd64.deb && sudo dpkg -i traceroute_2.0.18-1_amd64.deb
For i386 arch run
wget http://de.archive.ubuntu.com/ubuntu/pool/universe/t/traceroute/traceroute_2.0.18-1_i386.deb && sudo dpkg -i traceroute_2.0.18-1_i386.deb