Has someone already recommended 7dayshop.com? Best place in the UK to get film - really cheap and vat free.
As for the K1000, I picked one up off ebay last year for £50, in full working order, spotless 50mm lens, lightmeter accurate etc. Don't be too afraid to buy from ebay, even expensive items like cameras. I recently got my hassy from there too, and that was way more than the Pentax lol.
If you are going to buy from ebay in the future, just make sure they regularly sell cameras and have flawless feedback. Might want to check their refund policy as well.