I think this might be a little hot...
Purchase the 2100 Microsoft Xbox Live Points.
Purchase one 99p accesory from the list below. (of one of a higer amount... see my top tip!)
This will take the total over £20 and allow you to use the £10 off voucher.
Make sure you change the postage to the cheapest option. (£2.11 +VAT)
It is saying that it is estimated 3 days. Mine arrived in under a week.