Why Sea Dart was a poor air-to-air conversion

How plausible would an air-to-air version of Sea Dart be for a proposed British interceptor?

It would be a poor starting point; the AIM-54 Phoenix was already designed for the intended launch environment and mission.

The hypothetical asks whether Sea Dart could be adapted into an air-to-air missile for a proposed British interceptor. Sea Dart is a poor conversion starting point. If the requirement called for a large long-range air-to-air missile, the AIM-54 Phoenix would be the better choice. An available naval missile is not automatically preferable to a weapon already designed for the intended launch environment and mission.