In the interest of full disclosure, I'm affiliated with Proporta. However, that's a good thing because if you go here:
http://www.proporta.com/affiliate/Gadgetoid
And scroll down, you can pick up their antiglare protector (Advanced Screen Protector) for £13.45. (down from £14.95)
It's basically your pretty standard, good quality, antiglare screen protector affair complete with all the gubbins you need to apply it and smooth out the bubbles. I've had it on my iPad, and managed to apply it nearly perfectly (got some comments about it being applied well in the local Apple store), however I had to remove it (ARGH! sin of sins!) to test the Griffin one... which is pretty much identical in terms of feel and appearance.
Yes, I might be an affiliate, but I really do wholeheartedly recommend this protector... the commission I get selling these is a pittance anyway.
By the way, that price works out to £16.40 with worldwide shipping, and spending a further £23.60 will qualify for free shipping. iPad cases are slightly discounted too
Oh, and yes it does make the screen grainy/rainbowed... but it really doesn't bother me.