We've been waiting for it since what feels like the dawn of time, but Amazon UK has finally added the ability to give someone a Kindle eBook as a present.
Go to the book's Kindle page on the Amazon site (here's a good one, for example)
Find the button that says 'Buy for others' on the right and click it
Stick their email in the box if you want Amazon to send it directly to them, or just click 'Place your order' if you want to send it yourself.
Here's what you're looking for -- it's right above the wishlist dropdown:
The person you're buying for doesn't necessarily have to have a Kindle, either.