TL;DR: The company that builds the UK's nuclear weapons is hiring a junior (£33,300 – £45,000) and senior (£42,000 – £65,000) software developer, for roles related to "wargaming" and modelling the effects of nuclear weapons. The jobs are in Reading and you must be a British National to apply.

The Atomic Weapons Establishment (AWE) is the company that builds and maintains the warheads for the UK's nuclear weapons. They are hiring for a lot of jobs (177!), but there are two software engineer jobs (a junior and senior one) on the "Systems Assessments Battle Modelling Team" that jumped out at me as being relevant to people trying to build knowledge/career capital in nuclear security. Here's a snippet from the senior job description:

A unique opportunity has arisen within a dynamic and diverse team assessing the effects that nuclear weapons have on the offence and defence systems during a nuclear battle. The role includes the managing and development of complex computational models to enhance existing wargaming capability within the team and to run, analyse and visualise results from these models to inform key Government decisions for many years into the future.

...

The Systems Assessments Group (SAG) is a multi-discipline group, providing mission effectiveness and weapons effects assessments to AWE and our government stakeholders. We contribute a range of capabilities in support of the UK’s national nuclear security and treaty verification requirements.

 

The Battle Modelling Team within SAG covers a broad domain encompassing radiation transport, atmospheric physics and nuclear physics, mathematical algorithm development, operational analysis, Monte Carlo modelling and software development. The successful candidate will have qualifications and experience in software development and at least one of the other disciplines.

I'm not really sure about the ethics of working for a company that literally builds nuclear weapons, personally I would feel guilty about doing that. But, someone else would do it anyway and you are likely to be able to build career capital that would be hard to get elsewhere.