My wife and I each have an iPad Air, and have both had problems with wifi since installing iOS 8. I've tried rebooting the iPad, resetting Network settings, resetting the wifi router, and even resetting my iPad back to factory settings and setting it up as a completely new device. Nothing seemed to cure the problem of wifi and application freezes. It is particularly noticeable on the "TuneIn Radio" application, that I use a lot, that simply could not connect.
I too am using a BT HomeHub5 router, that the iPads automatically connect to the 5Ghz channel on. I have now changed the name of the 5Ghz channel, so I can choose whether to use this or not - well, it appears to have cured the problem!
If I connect to the 2.4Ghz channel, everything works fine; try the 5Ghz channel and it fails again. I've tried this about a dozen times, and it is consistent. We were both having to reboot our iPads at least every hour due to freezes, but my wife's has been fine for 48hours now on the 2.4Ghz connection.
I remain convinced that the problem is with iOS 8 - we didn't have this problem with iOS 7 - and hope that someone from Apple reads this, as it may help them identify the issue. In the meantime, this may help others like myself.
Best wishes