I put a GiffGaff SIM into my iPad running iOS 8 beta today and it seems like automatic downloading of carrier bundles actually does not work on the beta. So how do you get the right carrier bundle on there manually?

Download the plist which contains the list of all carrier bundles from here.
Search for your carrier and OS version in there, then get the value of the BundleURL key, in my case it was this.
Download that carrier bundle onto your Mac, because unfortunately just opening it in MobileSafari doesn't appear to work anymore on recent OS versions.

Enable "Carrier Testing"-mode in iTunes like this:

defaults write com.apple.iTunes carrier-testing -bool YES

Connect your device to the Mac and hit option-click on the "Check for Updates" button in iTunes. Browse to the .ipcc file you downloaded earlier and select it.
Your carrier bundle is put onto the device and you are good to go.