On a guilty plea the only thing that is used is a summary read out by the prosecutor.
the summary may well say about indications of how impaired you appeared to be, but it will be words, not videos.

You are first asked how you plead. Once you say "guilty." The prosecutor reads out the summary. The magistrates then ask if there is anything that you want to say. This is not on oath, or in the witness box, you just say what you want to say, or read out what you want to say, or indeed hand the court a letter. Your problem is that you MIGHT want to say you hadn't had a great deal to drink, and were confused because you ought that you could have legal advice before you blew.... but if the prosecutor sneaks in your roadside score then you only want to mention the second part. That is why saying what you want to say rather than handing in a letter would be the better option.

The magistrates might ask a question or two, but usually they just listen to your explanation, then consider what punishment to give yo?
if they do not offer a 'Drink Drive Rehabilitation Course with your sentence, ask to be allowed to do one. This can reduce your ban by up to 25%. It has to be offered and accepted on the day - but you do not have to do the course if you change your mind.

You could visit the court in the days before you are due in yourself to see how the process runs. Anyone can attend court as an observer so long as it is an adult court.