Bugger the weddingish cake. Ask them straight out if there's a colesworth handy where you can buy a packet of lamingtons before getting to the venue. You won't have a kitchen, the transport will be extremely anxiety inducing, and you've got no idea what facilities are available at destination for assembly/decoration. Not to mention the microbiology aspect of transporting food over such distances. In quantities appropriate for a wedding. This is a bloody thoughtless requirement loaded onto you. Lamingtons, packet lamingtons, is all they deserve.