You’re right that it’s speculative (the UK will retain the laws created by the directives into UK law as part of the “Great” Repeal Bill so nothing might change) but the UK could have higher standards already - the EU directive sets a minimum requirement, not a maximum.
Diesel-gate is a different issue and highlights the weakness of recognising other countries’ testing standards without setting sufficiently stringent regulations to ensure that testing was valid. The problem was that member states weren’t required to do their own testing but to trust that whoever did it had done it correctly while incentivising the testing state to be lenient lest they lose the payments to perform tests.
Diesel-gate is a different issue and highlights the weakness of recognising other countries’ testing standards without setting sufficiently stringent regulations to ensure that testing was valid. The problem was that member states weren’t required to do their own testing but to trust that whoever did it had done it correctly while incentivising the testing state to be lenient lest they lose the payments to perform tests.