The charger does not contain any lithium. It's a fairly simple bit of circuitry, mostly capacitors. Nothing unlike what you'd find inside your average computer power supply, and posing no risk of exploding due to an unpressurized compartment or moisture buildup like a lithium battery.
If this is a newer unibody MacBook, be very careful removing that battery. The underside is soft, and can be easily punctured. You don't want to find out the hard way the dangers of battery acid.
I'd also investigate other options for sending your laptop to the UK. Apple makes all their systems in China, and ships the early batches via plane. Unless they have a place somewhere in the US where they are installing batteries into these systems (seems unlikely), they clearly have some way of getting around those restrictions. And if you simply took the laptop with you on the plane, you shouldn't have any problems getting it through security, complete with battery.
