Hopefully this will help someone to save a little on this game. I've just finished Valhalla and skipped this but going to give it a go now.
The PS Store has the ultimate version of AC Odyssey on offer at the moment and if you top up your wallet £30 via instant-gaming you can get it for £25.52 and have £2.41 balance left for future purchases.
You can get this a lot cheaper through going through different countries stores, but if like me you want an easy method and keep to one account then I believe this is the cheapest way to do it at the moment.
Purchase the PSN card from the link, The code should be given instantly, Redeem the code via PS Store or on your console and then purchase the game from the funds in your wallet.
The PSN Top up card is HERE
The game can be found HERE