I've recently acquired a new 84-key US layout keyboard, which has fewer keys than a standard UK layout keyboard. I'm using it in EN-US layout to avoid being unable to type # but I'd still like to be able to type £ without using alt codes and would like to map it to [RightAlt]+. I've managed to map € to [RightAlt]+ using Ubuntu's built-in keyboard config tool but I'm a bit stuck on how to go about achieving what I want in Ubuntu 11.10.
I don't know the user-friendly way of doing this, so I hope you're fine with getting your hands dirty. :-)
For more information about Xmodmap, I would recommend Arch Linux's wiki page about it.