3G Data SIM Card (PAYG)

I shall be visiting the UK on infrequent occasions. As I want to stay in touch with my emails and access the Internet I have purchased a 3G Wireless Router with inbuilt modem (Solwise NET-3G-434T). This will enable me to connect my laptop (and my wife's Ipad) to the Internet. All I now need is a suitable 3G Data SIM Card to insert into the Router.

So, I am looking for a PAYG Data SIM Card.

I do not want to pay a monthly fixed amount. I simply want to pay for a SIM Card up front and then top it up (at an ATM or online) when I visit the UK. i.e. I want a simple PAYG system.

Can anyone help me please and identify a suitable 3G Data SIM Card?