Are you of any use to a consultancy? All depends on factors such as your attitude, flexibility, applied skills, etc. In and of itself, a PhD and 18 months work experience isn't going to make you a preferred candidate.You could look at firms such as Detica or PA Consulting who do a fair amount of work in and around the defence sector, so your work experience would at least be relevant.If your PhD is directly relevant to a particular client challenge, you might go in as a specialist consultant in that area. As you describe it as physical science, rather than engineering or ops research, that might not be the case. Therefore, you'd be looking at going in as a graduate hire, albeit with the prior work experience potentially helping you to get to grips with the work more quickly, so you can be promoted earlier.