Videos on YouTube are streamed through an Adobe Flash player. For the best viewing experience, we suggest you install the latest version of Adobe Flash after removing any old versions you've installed.
Please completely uninstall Flash before reinstalling the latest version.
To uninstall Flash, please follow Adobe's instructions, listed here: http://www.adobe.com/cfusion/knowledgebase/index.cfm?id=tn_14157
Once you've finished, restart your computer and follow Adobe's instructions for re-installing Flash: http://www.adobe.com/shockwave/download/download.cgi?P1_Prod_Version=ShockwaveFlash
If the instructions above don't resolve the issue, please try the following:

1. Temporarily disable any popup blocking software installed on your computer to test.
2. Allow uk.YouTube.com as a trusted site in your Firewall software.
3. Ensure other applications like Quicktime, iTunes, Real Player or Windows Media Player aren't set as the default streaming application, as this might affect the video player.

Failing all of the above download this browser...

http://www.mozilla-europe.org/en/firefox/