GMG is an authorized retailer and you'll receive the Shadow of Mordor Steam key immediately upon purchase. As an extra incentive,
GMG is also throwing in 4 free games + a 23% off coupon for future use.
This Shadow of Mordor deal from GMG is also valid in the UK/EU:
The Season Pass includes:
Pre-order bonus includes "The Dark Ranger" exclusive content. Includes a playable character skin plus "Test of Power" challenge mode. This bundle gets you the Season Pass too which includes a bunch of digital bonuses and additional content.