Once you've made your order, you should see the discount applied once you've completed doing so. Of course, you can use this on any other item that you would like to purchase (or if you DID want to purchase something for Mother's Day) but in my case, there wasn't anything that I really wanted or justified using the discount on until I discovered Halo 5 is probably one of the cheaper Xbox One titles to pre-order at the moment at £44.86 with FREE postage and packing in the UK.
Knock a fiver off and you're looking at £39.86. If anything like my recent order for Titanfall, the price MAY (and more than likely WILL) fall further down in price closer to release in which case, the previously applied discount will drop that price further down still (I got Titanfall for £32 with a previously applied £5 discount when the price dropped to £37.00!).
You follow? But again, feel free to use the offer on anything else that takes your fancy.