This is actually a divergence from the UK Government's position in its White Paper on the future relationship between the UK and the EU which stated that the Government's vision was to participate in these agencies:
the Government's vision is for an economic partnership that includes...
...participation by the UK in those EU agencies that provide authorisations for goods in highly regulated sectors – namely the European Chemicals Agency, the European Aviation Safety Agency, and the European Medicines Agency – accepting the rules of these agencies and contributing to their costs, under new...