Most cruise companies give you the option to either have them assign the cabin, or you can choose, at the time of booking. Often the ability for you to choose is an option once you have selected you cabin category and fare, and there’s a small supplement to lock a specific room.
If that’s not the case with P&O, and the cost and everything else is the same, sure… get the extra points! Make sure the TA is also offering the same cancellation conditions and fees, and any extras like onboard food credits, etc.
