Can I start answer section B first follow by section A for all P papers?

You can answer questions in any order. You have to write the question number you are attempting at the top of each page so it is clear to the examiner which you are answering.
Some people swear by answering question 1 first whilst others prefer starting with section B.

Whichever you choose, the key thing is to divide the time up between questions evenly. So including reading & planning time,1 hour 37 minutes should be spent on question 1 and 49 minutes each on your two questions in section C. Don’t fall into the trap of spending too long on your first two questions and only having a few minutes left for the third or not attempting it at all.