The IPC are much stricter in this regard - you can't even be a member unless your signs have been audited.
ParkingEye often use this as an excuse in court; they say their signs are 100% compliant with the BPA code of practice while knowing full well that the mandatory entrance signs are mising.
It looks like the DVLA are coming down on the side of the IPC with this one.
Here is an extract from their consumer forum meeting.
13. Concerns were raised about the lead time of three years for the introduction of mandatory
entrance signs. This timescale was seen to present a risk that motorists would be misled
and in turn the companies in question could have their access to DVLA data suspended.
14. BPA advised that it requests a programme of work from companies to show how they
intend to achieve the necessary changes within the three year period. The vast majority of
companies would have updated their signs well within this period. BPA also made the
point that longer lead times for parking sign changes were observed by the public sector.
ACTION: The BPA was invited to share the programme/s of work with DVLA
The minutes clearly state that misleading signage is a possible cause to have access to DVLA data suspended.
The Parking Prankster