Air is made up of a mixture of gasses. True, the biggest amount is Nitrogen (coming up for 80%) but there's also Oxygen (a little under 20%) plus Carbon Dioxide & a couple of others in very small amounts. Pure Nitrogen is illegal for use as a propellant in the UK AFAIK, but compressed air is just that - the stuff that we breathe compressed and cleaned. However, they seem to act in the same way as far as marker operation is concerned so if you have a standard air system then Nitrogen can be run through it without changing anything, & vice versa.
Maddogkwl - in summary you can bring your equipment with you as long as any fire modes other than true semi are locked off so you can't get at them (e.g. no burst fire or fully auto), but you will probably need a new bottle unless yours is dual tested to HSE as well as DOT and is new enough not to have needed testing.