Yeah you can just take it as extra luggage at a price, but note that you are supposed to officially import it UNLESS there is a certain amount that is duty free. Between most countries you can take a moderate amount of stuff with you without the need of importing it.
I hope you got the bill for it, if you dont youll have a big problem when you want to import it, mailing it to england would be a better option in that case, but then you also would need an adress in the US to post it from.
Youll get a guitar with a story in the end though, always nice i think.....