How is it decided which regiment/ corps you'll go into?

Hello. Thank you for your question.
If you choose a role and you meet every standard required, then you should be given the role you applied for, depending on the influx of other people also applying for the role, all that means is you may have to wait longer to start Basic Training. Some roles require a higher cognitive test score than others, some require certain fitness standards which are higher, some require an extra assessment at the assessment centre.
It will also come down to the interview you have at the Assessment centre, the assessors may not feel you are suitable for the role you have chosen and will discuss other options with you.
My advice would be is to make sure you meet the standards for the roles you have chosen and hopefully you will get what you want. Look at it as a long job interview.
I hope this helps
Zoe