A very young company with £700,000, a few employees and no trading history would get heavy "Know Your Customer" scrutiny at any bank whether or not it was defence or not. "Due to the nature of your business" could be due to that rather than it being a defence company.
They could well just have ticked to many boxes on Barclays "possibly suspect" checklist and got automatically denied by someone in compliance with hundreds of accounts to check a day.
Matt Levine/Bloomberg has talked about this a lot - where banks can turn the dial from giving everyone an account (and get sued and large fines - Barclays faced litigation due to Epstein), or reject more and get accused of not giving bank accounts to legitimate companies/people.
https://www.bloomberg.com/opinion/newsletters/2026-10-14/ban...