Trailfinders have very good deals especially if you are a bit flexible on your dates. I normally just ring them and ask what the best deals to my city are. They should have deals where you are able to get princes aroung £680-£900 pounds for that time of year.
I found Lee's travel online as well. They are based in England but you can book flights to Australia from anywhere in the UK and they should organise if your flying out from Glasgow as well.
Cheap Flights is also a price comparison sight but they seem to have good prices for flights to OZ at that time of year.
Hope this helps.