GameStop UK will sell you Call of Duty: Modern Warfare 3 for £0.99 if you trade-in Battlefield 3.
To enable the deal, buy Battlefield 3 before midnight on 27th October; then buy Modern Warfare 3 before midnight on 7th November; then send Battlefield 3 back so that GameStop receives it before 18th November.
If those steps are followed, your GameStop account will be refunded for MW3 minus £0.99.
That means you get around three weeks of Battlefield 3 play before you must send the game back.
Note that Activision increased the RRP of Call of Duty games to £54.99 back with Modern Warfare 2. Normal games sell for RRP £49.99, and shops usually knock at least £10 off of that.
That's why Modern Warfare 3 costs £41.97 at GameStop while Battlefield 3 costs £36.97.