Welcome to the forum.
You don't necessarily need the latest iOS update although it's a good idea to install the latest version of the app (currently 7.7). When you log into the website and go to My Account > My Passes
do you see your 14 day free trial showing correctly. If you do, are you sure you are in the UK (ie it won't work if you are on holiday abroad). Assuming you are try turning off your router at the mains for 10-15 mins and then reconnect and try again. You could also try switching to mobile data on your phone and seeing if you can play something now (just play a second or two to check if it works otherwise you'll eat up your mobile data allowance).