Waiting for adequacy
What nobody here (or apparently anyone in government circles) has noted yet is that loss of access to these national databases is only a part of the data processing debacle.
The EU position that an adequacy may be made during the transition period automatically implies that for quite some time between March 30th and the making of a decision British businesses will face serious complications, or indeed may be barred from, transferring personal data out of Europe and quite possibly also from processing any such data received if the processing involves onward transfers to other third countries.
In the worst case, the European party to such transfers will be at liberty to terminate the arrangement, and at best UK businesses will have to enter unfamiliar negotiations with the European side and undertake to fulfil novel responsibilities that nobody has yet explained authoritatively to them - and this with a mere four months to go before Brexit.
I've been pressing the authorities for quite some time to provide explicit reliable guidance before it's too late on what UK businesses have to do to ensure continued lawful transfers of personal data from the EU. But it could already be too late for many, particularly in the SMB sector where my professional experience shows that the entirety of data protection is still much of a mystery. So the inertia of the government in this could quite easily put small businesses out of business or force them to operate unlawfully in order to stay in business.