Gears of war 4 is currently £40 in Tesco however I just went in-store (Tesco Extra - Cleveland) and they had £10 off if you buy a valid currency card.
The £10 one didn't work although the £15 one did.
This means you get GOW 4 and a £15 Microsoft card for £45!!!
Considering GOW 4 is £40 on it's own, £5 for a £15 credit is excellent!
If you don't want it, you could always sell it online for at least £10 :)