Are they Mac specific? I only ask because for an HP laptop you could get a UK plug on Amazon for about a pound.
Actually, I can't remember as it's been a while since I looked - as it works fine with the adapter, I just haven't bothered. Looking on Amazon now, it does seem like you can get fairly cheap ones.
I have had to replace the brick part of the power cord twice now - the first time, Apple replaced it for me for free as it was still under warranty, but they only gave me the brick part of the cable. The other time I ordered one off Amazon, but again it only came with the brick half of the cable. I do have a UK plug that goes straight into the brick, but not a longer cable with the UK plug on the end.