Its simple. register with your 7 digit ID and password. Sip server is: sipgate.co.uk
also try
Business Call Management Solutions | Orbtalk for payg. Orbtalk did issue free number but I am not aware of current situation. Betamax certainly issue voip-in numbers once credit bought ,however the company has been issueing 0560XXX..... numbers recently and not geographical.
Third one is Terrasip.