I have used a headphone splitter with an iPod Touch and it worked just fine. We were both using genuine Apple earbuds so I don't know if that made a difference or not as far as being the proper impedance.
As far as getting a splitter at Radio Shack, they certainly do have them, but since the OP is in the UK, I do not know if there even are any Radio Shack stores over there any more. Rumor has it they have one foot in the grave over here on this side of the pond.
"You have questions, we have cell phones!"