So....
If you fill growlers out of a 30L keg you have bought off the brewery, the excise has been paid by the brewery, and you dont need to pay excise.
If you fill out of a 50L keg, you will need to apply for an excise licence, and you will need to pay excise on what is sold as you are filling from a vessel under 49L.
Also, imported kegs have already had the excise paid I believe, and regardless of size you pay no more.
SO
To get around it as a venue, only pour out of 30L kegs/ imported kegs or pour out of 50L and hope no one says anything to the taxman or pay the excise like a good law abiding citizen, although I hear they are cracking down on it down south if you dont.
Hope that helps.