I know the Tesco £13.97 for a Xbox 360 Live 2100 Points Card has come up before, but a glitch means you can get 4200 for a better deal.
Tesco will charge £1.99 for delivery of both items. When using the FREEDEL code to get free delivery, the voucher will give you £3.28 off, bringing the price down to £13.33 per card (£26.65 in total)
The more you purchase, the more it seems to give back on delivery.
EDIT: This is on Tesco Entertainment (just to clarify)
And 7% Qudico