Is it possible to prequalify an installation and start providing before the relevant compliance test is completed for the installation?

No. Entirely new installations connecting to the grid must undergo a process known as compliance testing. This process tests that the installation will not damage the grid, such as in the case of voltage drops. This is a requirement in order to sign a connection agreement with the grid owner in question

All connection requirements must be met, including compliance testing, and the connection agreement must be in place before the prequalification process can begin.