I just wanted to add a little more info on this problem.
I can personally verify that 3 different customers on Virgin Media cable are experiencing problems with League of Legends. We are seeing 2-5 second lag spikes every minute or so, rendering the game unplayable.
Please see this thread for more players with issues:
Hard freezes every minute, for a period around 5 seconds, rendering game completely unplayable. Lack of a response from a representative is slightly disappointing, didn't expect much however. Hopefully we can get a word from both ends on the matter within the day.