GMG just extended the coupon again! (There's a new coupon on GMG's site but it didn't work for ESO, happily the old code is still working. According to email, it has been extended until this Friday).
ESO Pre-Order Price for UK and EU
Pre-Order Bonus + Imperial Edition Bonus
When you pre-order the Imperial Edition from GMG (or any other retailer), you'll first receive your early access key which you can use to register and sign up for your account at ESO. This will net you the early access and Explorer Pack. Prior to release date, you'll receive another set of key which will be the key for the actual Imperial Edition. Most publishers, including Bethesda handles pre-order bonuses this way.
ESO Pre-Order Price for UK