Plessey Semiconductors Limited is strategically significant not because it produces finished defence platforms, but because it occupies a more foundational position in the technology stack, where compound semiconductors, microLED devices, and photonic components shape the performance envelope of advanced military and dual-use systems. Its relevance to European strategic autonomy lies in its potential to supply high-value enabling technologies for displays, sensing environments, and energy-efficient computing infrastructure at a time when defence resilience increasingly depends on secure access to specialised microelectronics rather than on platforms alone. As a UK-based manufacturer with verifiable strengths in GaN-on-silicon and microLED-related industrial capability, Plessey represents a technologically relevant but institutionally complex case: close enough to the European security ecosystem to matter, yet outside the EU legal perimeter in ways that sharpen the distinction between capability value and procurability under autonomy-oriented frameworks.