Use a £20 voucher from cdkeys and use 5% to get it cheaper. http://www.cdkeys.com/top-up-cards/steam/steam-wallet-top-up-20-gbp
Rise of the Tomb Raider: 20 Year Celebration includes the base game and Season Pass featuring all-new content. Explore Croft Manor in the new “Blood Ties” story, then defend it against a zombie invasion in “Lara’s Nightmare”. Survive extreme conditions with a friend in the new online Co-Op Endurance mode, and brave the new “Extreme Survivor” difficulty. Also features an outfit and weapon inspired by Tomb Raider III, and 5 classic Lara skins. Existing DLC will challenge you to explore a new tomb that houses an ancient terror in Baba Yaga: The Temple of the Witch, and combat waves of infected predators in Cold Darkness Awakened.