I really like the 2009 / 2010 syllabus for Grade 8 and would like to buy it even though I am a way off being able to play it. Will it be discounted when it is no longer current (in which case I will wait a while) or will it disappear and I would be better off buying it now?

Yes it will probably be discounted as I bought the old grade 6 and 7 books (sorry can't remember how long after they'd expired) for £1.95 each.

Agreed, and even if you can't find it in the music shop after it's no longer current you should be able to find it on eBay or Amazon.