Well, first the usual question, what does your teacher think? He or she knows you and your capabilities, and can advise more knowledgably than a stranger.
Next question, who chose the Poulenc, you or your teacher? If your teacher usually chooses pieces for you, it might be worth asking why this particluar one.
Have you had a chance to listen to the other Grade 8 List B pieces yet? If not, then it might be an idea to listen to as many of the options as possible, as there may be a piece you would really love on the list. It would also be a good idea to listen to a recording of the Poulenc if you haven't already done so, as it will help to get an idea of the overall shape of the movement.
The opening few notes are very tricky to play well, because the intervals between them require a lot of precision in the embouchure. There are some other challenging technical aspects to the movement (like the infamous double-tonguing) which need patience and persistance to perfect. It may be that another piece would suit you better, but two weeks is not very long to have worked on a piece of this complexity.
If you do change to another piece, then you might enjoy the Mouquet Flute de Pan, where you have two movements to choose from, or the York Bowen Andante piacevole.
Finally, I (a long time ago) was in the same position as you, and switched from the Poulenc (which I deperately wanted to play) to a different piece for my Grade 8. I ended up with a distinction, so it was the right decision for me. Best of luck with your choice.