Get a new GiffGaff SIM card (they're free) using my referral code, add some credit (pay as you go) or get a plan (plans can be monthly so you can just quit your plan whenever you want - no contracts).
When you activate your new SIM (you'll get instructions with your SIM card on how to do this) and add your credit/plan, you'll receive an extra £5 credit. This is usually applied immediately to your account.