Well, a perfectly legal way to watch it is wait until the DVD comes out, or the BBC show it (assuming they do, like they did with the first movie). I will also say it has popped up on YouTube a few times, but they are very quick to remove it due to obvious reasons.
Alternatively, assuming you have either Sky or Virgin Media, you could treat yourself and upgrade to get Disney Channel for the month. On Sky it's part of the KiRAB Mix.