Held off posting this until I received and sent my items, now that it's out of the way I'll share.
Amazon are currently running a promotion where they give you an extra £5 credit when you trade in £15 worth of games.
I searched for a few games to trade in to get the most out of this and I stumbled upon NHL 14 which can be traded in for £15.50 and and can be bought for about £10 from Boomerang rentals, so trading in two would get you £31 + £5 bonus = £36 for a £20 spend.
You can also trade in Deadfall (Link in first post) for £9, this costs you £5.99 each (2 for £11.98 total and you get bck 36p in points) and you will get £18 + £5 = £23 in Amazon vouchers.
Amazon even offers you free post via Royal Mail or Collect+.
Feel free to post any suggestions if you've found a better spend to voucher ratio.
You can only trade in a maximum of 2 of the same game per account; you can also only get one £5 voucher per account.
CHECK YOUR CARD ACCOUNTS THAT YOU USED FOR YOUR ORDERS; IT LOOKS LIKE BOOMERANG WERE HACKED.
REPORT ANY FRAUDULENT ACTIVITY.