Amazon currently have the Veho Pebble portable battery for only £35.77.
For those of you who don't know what it is, it's essentially a USB backup battery that can charge any USB device (requiring less than 950ma). It comes with a bunch of cables, but you can also use any USB cable that comes with a device you own.
The 5000mAH capacity is enough to charge the iPhone 3G 4 times before it needs to be recharged itself.
(Note: This will not charge the iPad, as it requires 1 Amp when turned on - many people report that it will charge fine with the iPad fully turned off, though)
Edit: This price is a result of a great amazon price fight with a marketplace seller - where it seems to automatically reduce amazon's price to compete!
Edit 2: Dropped even further! 34.74 from Amazon.