You need to full retail copy install DVD of OS X 10.5 Leopard. No grey colored discs.
While OS X 10.5 may not be too hard to obtain, it may well be fairly expensive to obtain, now.
That OS X version is six years old, now.
If you have any other authorized Apple resellers in the U.K., besides trying eBay and Amazon, they may have older full retail copies lying around for sale.