If you get it chilled right down and then wrap in grease proof, followed by cling film nice and tight, and then a couple of layers of insulating and protective bubble wrap, before putting in a as snug a box you can find and pack the corners with polystyrene chips or screwed up newspaper I think you'll be fine.
Take it to the post office as close to collection time as you can as that is probably the only time it is going to be anywhere warm is hand send it using a guaranteed delivery time, and write 'fragile' on the box so that the pasties know to use it as a football ;-)