I've tried this and it still forces SSL. I don't have an SSL for my domain yet but I can see this working a treat. Thanks for that.
It appears to be problem with WHMCS forcing SSL.
I put in a ticket and they replied with:
"Thank you for contacting WHMCS support! It appears that the remote WHM server itself is configured to force requests over SSL without a valid SSL certificate, and you would need to disable this on the server to resolve it. I believe the settings for this are under the Redirect tab on Tweak Settings, but your server admin or cPanel support can check and help with changing them as needed."
Both me and my reseller provider can confirm that this isn't the case. We have not set it to force SSL and the fact that it worked pre-6.0 shows it's a 6.0 problem... Annoying.