The reverse bay park manoeuvre come test seems to only happen on DVSA property in Bristol. I can't speak for all test centres nationwide. I suspect it's due to being more time consuming plus having a higher degree of difficulty and risk. Therefore you'd expect this at the very end of the test, although I have seen it occur right at the start too.
This blog post is for non-driving instructors to print out and use to help their friends and families, correctly perform the reverse bay park manoeuvre.
The examiner will stop you in the car park and will ask you to reverse the car into a bay of your choice, paying due attention to other road users, finishing between the lines. You can always fix things if you're wonky at the end. Don't worry about cutting the lines a little.