So to sum it all into one easy post:
You cant use a US stamped bottle in the UK.
Any HPA (High pressure air) or N2 tank is EXACTLY the same thing. An air tank will hold air or N2, but a CO2 tank is a completely different beast, and you cant put CO2 through an Ion (Well you TECHNICALLY can, but its really not advised)
Getting N2 fills is expensive and near impossible, air is the way forwards.
Buying something from the USA means you will have to pay for shipping, import duty (about 3.5% i think) VAT, another 17.5%, and if something goes wrong, it going to be a bitch to return it.
You can buy an Ion in the UK from any of these websites, or alternatively buy a 2nd hand one for VERY cheap off of the forum you are reading right now:
www.planeteclipse.com
http://www.lpspaintball.biz
www.justpaintball.com