Correct height difference between DPC overhang and drains

Given the height of the gulleys, I'd say it was proposed to be finished in concrete on the gravel but it was never finished.

Why would fitting EWI below the existing DPC be more of a thermal bridge? The DPC has nothing to do with thermal bridging. There are other ways to fit the EWI which incorporates DPC to prevent moisture ingress, while still extending the EWI low enough to reduce thermal bridging through the structure.

All this depends on the system/EWI being used, and manufacturer's/installer's instructions.

I think some form of DPC would still be required, though if the existing wall is a cavity wall, it's likely any moisture which rises up through the external insulation and over the existing DPC would come back down through the cavity, so it's unlikely to ever travel to the interior and cause an issue. But even then, the role of the DPC is nothing to do with thermal bridging, and the bigger issue would be having no external insulation at plinth level (under the DPC), because that gives too much of a path for actual thermal bridging (which in turn can cause lower surface temperature at the base of the wall, which leads to condensation forming, which leads to mould and degradation of finishes).

It's not an either/or situation, there are still ways damp-proofing can be incorporated into the EWI to minimise or fully negate the risk of moisture travelling through the structure.

The term Thermal Bridging is just a very particular thing. What you're talking about with damp getting through the structure due to gaps in damp-proofing, while part of the issue it causes is thermal related (though I would say it's a minor part of a much bigger issue), it just doesn't constitute thermal bridging, which is a term related to the excess passing of heat through part of the structure due to inadequate insulating materials (not the actual definition, just the phrasing that came into my head as I typed).