I went in the Birmingham store today and they had a table with red cards on it. It costed nothing to pick a card, and if you got a lucky one you won £20 to spend in store. I was in fact lucky and did!
I don't know if this is national, or was just for today, but maybe is worth popping into your local store as they will let you have a go for free at picking a card - and you might get lucky too! They also printed a free polaroid picture of me and my lucky card, for free! :)
Terms: (I've summarised them)
1. UK residents 18 and over (although they didn't really check when I was there I don't think, apart from giving a postcode perhaps)
2. Dates this is on are the 12th, 13th, 18th and 19th of December 2009 (but who knows if they might do days in between.
3. "There are 50 prizes of £20 T K Maxx gift cards to be won each day."
4. A person can only get a photo and win once a day (getting a photo gets you the red card, and then you see if you've won... however, I was able to pick the card first...)
5. Giftcard has to be redeemed in one transaction/go.
6. No purchase necessary (aka FREEEE!)
7. MUST BE REDEEEMED by close of business 24th December 2009.
Good luck all! I know I'll be heading back on the 18th/19th! :D