Our biggest advice is to be honest. If you feel there were errors in your understanding of the policies and/or you may have accidently (or intentionally) completed academic misconduct, it is best to admit this as early on in the process as possible.
If you don't feel there were any errors, try and remain calm and respectful in your response. Explain how you went about preparing the assessment and were mindful of the regulations surrounding academic misconduct throughout your preparation. If you have any of the notes you made alongside writing your work which help to show the work you put into the assessment, these could be helpful to send as supporting evidence to the Proctors.
If you were experiencing any mitigating circumstances, such as personal difficulties (including bereavements, relationship breakdowns, victim/survivor of a crime) or medical difficulties (including mental or physical health challenges) then explain these. You don't need to go into lots of details, but a brief summary of what happened and the impact it had on your ability to fully engage in your studies is helpful. You should provide this context whether you are admitting to the allegations or not.
For example, if you were experiencing depression you might say: 'In the month leading up to and including the submission of the assessment I was experiencing depression. I had difficulties sleeping, with my appetite and focussing on my work. This meant I was unable to focus fully on my studies.'
Ideally, you should provide some evidence to support the challenges you experienced.