Hi Aidan
Have you tried Metal Supermarket in the UK. There's quite a few scattered around the country and they will cut pretty much anything to size. However, it's worth telling them you want the metal kept flat. They have some HUGE guillotines that tend to bend the metal slightly unless you mention to them to be careful!
I saw some alu plate on one of their shelves that was probably 20mm ish, don't know how much that would cost though, pretty expensive I guess.
Jem