Grab yourself a free sim from Lebara with £1 free credit included. That means you don't even need to topup to get it. They're a fantastic network with no contracts. They offer competitive national & international call rates worldwide so you could save a fair bit of money by choosing Lebara.
After ordering your SIM online, they can transfer your current number and you get £4 further free credit.
You can manage your Lebara account online anytime day or night, whenever it suits you. To get your free SIM just choose the top option on the page linked and create your account with them. It couldn't be easier.