BAE Systems does have capabilities in both the UK and US, but the development of its electromagnetic railgun technology-most notably for the U.S. Navy-has been concentrated in the United States for several key reasons:

1. Primary Customer: The U.S. Department of Defense

• The railgun project was developed for the U.S. Navy, which provided the majority of funding and set the operational requirements.
• BAE Systems, though headquartered in the UK, operates a large and relatively independent U.S. subsidiary (BAE Systems Inc.) that handles most American defense contracts.
• U.S. military R&D budgets are significantly larger than those of the UK, allowing for long-term, high-risk technology development like railguns.

2. Regulatory and Security Considerations

• Advanced weapon systems like railguns fall under ITAR (International Traffic in Arms Regulations), making collaboration across borders complex and often restricted.
• Developing the technology within the U.S. simplifies security, export control, and collaboration with U.S. government labs and test facilities.

3. Facilities and Infrastructure

• The U.S. has the necessary test ranges, high-energy labs, and funding to support such high-power electromagnetic weapons research.
• Key development and testing occurred at U.S. Navy facilities such as the Naval Surface Warfare Center in Dahlgren, Virginia.

4. Lack of UK Demand or Funding

• The UK Ministry of Defence (MoD) has not issued comparable requirements or committed to significant investment in railgun technology.
• Without a UK-based customer, it would be impractical to develop and sustain a program of this magnitude in the UK.

5. Strategic Business Focus

• BAE Systems operates with regional autonomy, meaning BAE Systems Inc. (US) focuses on U.S. contracts, while BAE Systems plc (UK) focuses on British and European projects.
• High-end electromagnetic weapons fall squarely within the U.S. military's priorities, unlike the current UK defense posture which prioritizes other domains (e.g., cyber, AI, hypersonics).

Summary

BAE hasn't developed railgun technology in the UK largely because the funding, demand, facilities, and legal environment for such advanced weaponry exist predominantly in the United States. The U.S. Navy was both the sponsor and the intended operator, making it natural for development to occur stateside under BAE's American arm.
If the UK were to shift priorities and invest in such technologies, BAE could potentially leverage its experience to develop similar systems in the UK in the future.