I'm in none of these categories

• if you are coming to the UK as an academic visitor you do not need an ATAS certificate.
• if you are coming to the UK to do research that is not linked to any studies you do not need an ATAS certificate.
• If you are applying for an extension to stay for the sole purpose of writing up your thesis and your previous Visa did not require ATAS clearance and was issued before 30 November 2007, then you don’t need an ATAS certificate. If your Visa was issued after the 30 November 2007 then you will require ATAS clearance for writing up your thesis.
• if you already have a valid visa you do not need to apply for an ATAS certificate unless you need to apply for an extension to stay in the UK in one of the above categories. 
• you are studying a PGCE or Postgraduate Diploma.

Details on the Immigration Rules for any of the above categories can be found on the UK Border Agency website.