As of this morning,
https://www.apple.com/uk/retail/ is stating the iPad Air 2 is "now available" at the Apple Retail Store ("*supplies may be limited").
There's no reservation system or details on whether the cellular models will be available but I'm going to my local store this morning. If they have the cellular models I'll be cancelling my web order (obviously).
Have to say I'm disappointed with the roll-out of the new iPad no announcement of the launch date, odd online preorder time, inconsistent delivery estimates, etc..