Thought this was probably the best place to put this, but for anyone interested, the Season Pass for Borderlands: The Pre-Sequel can be obtained for as little as £4.99 from GAME.co.uk.
The below link is for the Mac+Linux version of the Season Pass, but being a Steam Play game, if you own it on the Mac platform you get the Windows platform too, and vice versa. I was a little wary of trying it myself, but it worked great. Got my code emailed to me almost immediately and it activated on Steam fine with my Windows version installed. http://www.game.co.uk/en/Borderlands%3a+The+Pre-Sequel+Season+Pass+%28Mac+%26+Linux%29-317638?cm_vc=Prod_1
What is strange is that they also sell the Windows version of the Season Pass for £24.99. So, this may be an error on GAME's part and I've no idea how long it will remain available.