Q: Does 3G / 4G on the iPad Air auto turn of when its ran out of data?

Depends on how your wireless provider has it set up. Some will cut you

off and send you a warning that you need to buy more data, others just

let you consume as much as you want and bill you for the overage. There

is no function in iOS that can be set to stop the connection after a set amount

of data has been used.

Check with O2 to see if how their plan works and if they provide warnings before

you exceed your data cap.

Learn about cellular data settings and usage on your iPhone and iPad (Cellular Model) - Apple Support