This is most definitely an IOS8 issue - and whilst the 8.0.2 patch makes the iPad more stable, it does not fix the fault that kills 5GHz connections. I have exactly the same issue whereby 5GHz networks cannot be used with the iPad subsequent to upgrade from IOS 7.1.2 to IOS 8 - or IOS 8.0.2.
I have completed tests that consistently prove that the problem is nothing at all to do with the ISP (BT) or the router (also in my case an BT Homehub 5). At present, I have a recent iPad Mini Retina (wifi & cellular) and iPad Air (wifi & cellular) purchased same day. Both have been running IOS 7.1.2 without issue, with various 5GHz networks - including the domestic HH5 router and business networks My domestic network also has an AppleTV and PC forced onto the 5GHz network, also working without any network issues at all.
The domestic router has 2.4GHz & 5GHz networks configured thus:
5GHz: SSID = NetworkName-A, WPA2
2.4GHz: SSID = NetworkName-B, WPA2
The iPad Mini Retina IOS 7.1.2 would interoperate with both 5GHz & 2.4GHz networks without issue, both domestic (above) and other networks such as that at my place of work. After "upgrade" to IOS 8, the iPad is unusable with any 5GHz networks; it will associate, but will not exchange network traffic for more than a few initial seconds - after which networking app's such as Safari and email will hang. All other devices (including non IOS 8 Apple kit) continue to operate completely normally - as will the iPad Mini Retina if is forced onto a 2.4GHz network.
The iPad Air (still running IOS 7.1.2) continues to operate with the same 5GHz & 2.4GHz networks without any problems at all. Given the issues experienced with IOS 8 & IOS 8.0.2 - and as consistently described by others within this forum - the iPad Air has not (and will not) be upgraded until this serious network connectivity issue is resolved by Apple.
All of my attempts to resolve the IOS 8 iPad 5GHz networking issues have failed, escalating from "forget network" to "reset network settings" and "reset all settings". Restore doesn't fix the problem either.
For those affected by this serious network bug, the only apparent workaround (but not fix) that might make your iPad semi-usable is to utilise different SSIDs for routers that you are able configure - and only configure the corresponding 2.4GHz network SSID on the iPad. If your router or wifi network utilises a common SSID for both 5GHz and 2.4GHz networks, client connection strategies will usually attempt to preferentially connect to the 5GHz network where both are available. Unless you can force your iPad to only use 2.4GHz, with IOS 8, all kinds of issues will ensue as the iPad will not tell you which "band" it is attemping to utilise - and available settings will not allow you to explicitly disable either band.
Come on Apple - you can replicate the same test conditions. Admit and resolve this issue. I can appreciate you concentrating on the launch of the new iPhone 6, but your new IOS 8 update has introduced a major network bug in most other iThings.