M8,
Technically the BRP needs to be updated once you have the changed passport to keep it in sync. For now you can travel to pakistan on the passport with Maiden name and also the BRP card can be used to enter England.
I think you are working yourself up overtly. They will not impose any fines if you are travelling to your home country as the British naturalisation is being processed. Remember once you get the British Passport you wont need to use the BRP anymore!!
cheers
vr
PS: I am not an ''Experienced member'' but trying to gain experience at your expense!!
