You can donate to the Tsunami disaster fund via Amazon.co.uk which makes things a bit easier as I have found the dec.org.uk site down a few times.

I've contributed through here. Just make sure you click the tickbox at the end which means they get the UK Tax relief as well.

If you want to see the awesome power of the tsunami there are some videos on this link: Tsunami Videos, that should make you think about the damage it has caused to lives and property.