Hi,
If you always want the number barred the quickest solution I would assume would be to go to:
Trunks > Digital > ISDN > Outgoing Call Cha.... > Calling Line ID Restriction and change it to restrict. If the provider lets the PBX decide the calls should be "blocked/private" on external devices.
UK uses as far as I know Euro ISDN(30) so I would think that solution will work fine.
If you want to define it on a per call basis let me know because that is a bit more text and work
Cheers,
Dutch