I think it's fine and smart to use pressure treated timber for this purpose. The treatment repels insects and wood decay and maybe some vermin. It's still organic.

Well, unless you buy a metal or plastic* shed (*not really organic either) you have little choice but to use pressure treated wood - if you don't, you'll be replacing it every couple of years. Potting sheds are just sheds, primarily a place to work out of the rain and maybe keep small plants, but you can't keep plants in them for any length of time - if that's what you wanted to do, get a greenhouse instead.

I often wish I had bought a polytunnel, as more space !
Being organic is keeping the soil in a natural state without
adding chemical nitrates as farmers do - only they dont tell us in their T/V interviews.
There are books in every library on Organic Gardening.
Cheapest way to study the subject, this winter.

Most pressure treated wood contains arsenic, which should not be allowed to leach into the soil. I would build the shed on cinder blocks--not certain what they are called in the UK--where the lime in the cement will bind the arsenic.

For some reason Cinder blocks are called Breeze blocks in the UK Tug.....no idea why. .......

Not in the UK or Europe, Tugbrethil - restricted in 2003 by the EU and banned by January, 2004. Which means any new wooden structures sold in the UK now have no arsenic content... I find it paradoxical that the EU provides more protection for British citizens than the British government does, and thank the lord for it... link below for more info

http://www.barlowsofhermitage.co.uk/media/23750/some%20facts%20on%20chemically%20treated%20wood.pdf

Just do not use pressure treated timber where it will come into contact with edible crops or livestock - raised vegetable beds, etc.