You must buy a special can / bottle which has a coke zone code (guessing these could cost 50p?). Then go to cokezone and sign-up. Signing up gives 2 points, fill in their monthly poll for 2 more and then answer their additional questions for 5 more points. Enter your code from the can and you will have 10 points. Spend it on a £5 voucher at ea store (link: http://tinyurl.com/y3lenr6)
Once at the EA store choose any PC games for under £10 and add as many as you like to your basket. After adding each, type in your £5 voucher code. It will deduct £10 from each item in your basket. Then head to checkout! No need for credit card details (if it wants you to enter them, go back one page and re-enter the voucher code). Done!
Some games that are available
Battlefield 2: Special Forces
Tiger Woods PGA TOUR® 08
Need for Speed Carbon Collector's Edition
Black and White® 2: Battle of the Gods (Expansion pack)
NBA Live 08
Madden NFL 08
Mirror's Edge Pure Time Trials Map Pack
Mass Effect Pinnacle Station
Medal of Honor Airborne
SPORE Creature Creator
If you are having problems getting it to work try it on google chrome - works for me on that!