TXwhich personal allowance for September 2019 sitting

You have to use the figure used in lectures. The Finance Act 2018 is to be tested in June 19 sitting, which is also the basis of lectures. You can also confirm it from the following article published by the examining team. https://www.accaglobal.com/pk/en/student/exam-support-resources/fundamentals-exams-study-resources/f6/technical-articles/finance-act-2018.html

Your examined under the FA 2018 so 11850 is what you will use