Can you try to enter the 141 first before dialing the outbound call?
There are 2 items that need to be verified.
1) Did the UK region pack contain the 141 code for blocking the outgoing CLID on a per call basis
2) the 141 must be dialed first on a SPA phone and then the called party number. The SPA phone appends the feature code in the outbound call.
Please try that and update if you have an opprotunity.