I’ve graduated aged 21. Can I go straight into teacher training?
Some lucky people know very early on in life exactly what they want to do. Some of them even want to be physics teachers!
There is nothing to stop you taking an undergraduate degree then immediately applying for ITT. You could easily be standing at the front of a school physics lab passing on your passion by the tender age of 22.
Similarly, you may decide to continue your studies as a graduate before deciding to teach. Alternatively, taking a few years out exploring other options before applying will not prejudice ITT admissions tutors against your application.
What they will be scrutinising your application for is evidence that you know what being a teacher is actually like. Have you spent time observing science lessons in a school? (link to SEP) Have you spoken to physics teachers and understood the reality of the job?