You do, it is £29, plus p&p. It isn't tied to one network, it is a 'roaming sim card' as they call it. It picks up what ever network is strongest/they have an agreement with.
I have only just got mine. It isn't all that cheap (maybe slightly less that roaming with your normal sim card) for texts, but for calling it is pretty good, as long as you are calling a landline.
I mainly have mine on so that I am contactable by friends and family if they urgently need me.