With O2 Priority you can get £1 off a meal deal at Boots today. It normally costs £3.29 so it'd be £2.29. In London it costs a bit more.
You can get a sandwich, pastas, salads and graze boxes, crisps or chocolate with it, plus a soft drink.
If you don't have O2 Priority you can get a free sim card at O2. Also, remember to only push the button when you're at the till.