O2 is still giving away free SIM cards. These come with 300 free texts a month if you top up £10 a month but will work fine (without free texts) without a monthly top up. If you haven't gotten in on this deal yet jump on it now! You can apply for as many as 6 cards per house.
Where: O2 Promotions
How: Fill in the shipping info and get up to 6 cards within 28 days.