I'd double check the small print first. Usually it tells you the amount of each prize they have, and usually the most is some part vouchers (Like the Thomas Cook ones you mention. They'll give you a 'free' night or somthing somewhere but you have to pay for the flight on top, or insurance, or for an extra few days..., ect.) or a cheap necklace. It's also usually a premium rate number you call which will cost £1.50 a min and probibly total £15.00 or more.
I know you are trying to do a nice thing, but as suggested, check the small print first.