The real problem is you can't _have_ a legal copy of Mac OS X 10.4 (Tiger) for intel Macs unless you've bought an intel Mac. That OS does not exist in a retail version so far.
If you're going to try PearPC, which slowly emulates a PowerPC Mac, you won't get a real feeling for how a Mac really is and feels. So if you truly intend to get some experience with Mac OS X, there isn't really a way around buying a fairly new Mac, which isn't too expensive. I'm sure you'll find an almost-new iBook G4 or some form of Mac mini around the UK for not that much money. If you get, say, a MacBook, you can even retire one of your old PCs, because the MacBook can run Windows XP natively as well.
Mac user since 1987. Running Mac OS X 10.8 Mountain Lion on a MacBook Air 11" & an iMac 27" and whatever's newest for my iPhone 4s, iPad 3 and AppleTV 2.
Apple Certified System Administrator 10.6, Apple Sales Professional 2008-2011, Apple Certified Mac Technician.