I'm looking into this too. The problem with asking about tanalised is that most suppliers respond with "Oh yes, they're properly pressure treated" but then won't offer any kind of guarantee.
My local supplier can do 1/2 round (pressure treated spruce) posts for £1.15, but they're not guaranteed and the local farmer who suggested the supplier admits they don't last.
I did find this almost local supplier:
http://www.fenceandforget.co.uk/index.htmlbut they're £3 a post
(though the posts should outlast me) and another in Bath (Jacksons) that guarantee for 25 years and will deliver to SW wales but by then it works out at about £3 a post again.
I think I may buy cheap posts (and make some from overgrown hedges) and stand them in creosote 'til they soak it up above ground level. Trouble is, that takes time, especially for the home-made ones that are not dried yet, and I want to get some fencing done whilst the ground is wet rather than wait for summer.
maybe I will just have to get my wallet out
mab