There is no need to ask the seller if they use the GSP as it is clearly shown on the listings unless you have logged into the Us or UK sites. 

If you are browsing through the Australian site you will be told that the GSP is being used.

Many sellers have no idea they have been opted into the GSP.....they often do not sell internationally and it is only be ebay opting them in that you can even buy from them. 

The fact that you saw free postage on a UK item but were charged for the GSP indicates that the seller was not intending to ship internationally but was opted in by ebay.....they still post via free postage to an address in the UK but you pay the high GSP shipping rate.   Did your seller realise that you are not in the UK.....did you make it clear that you are in Australia and he woulds be paying more for postage than just a local location?

What I don't understand is why an A4 sheet of paper is being sent with the GSP as it is only meant to be for parcels.

Nobody likes the GSP as you would know if you read the boards.   There are plenty of threads about it and all it's failings which have nothing to do with the sellers in most cases.   However it is very cost effective for large and heavy items but this depends on sellers listing the correct weights and measurements of the item and packaging.