The way I understood it is: If flying a flight test with an examiner and you pass, P1/US is the norm. Anything else with an instructor is PUT. Ironic really, I did my JAR bienniel with an examiner, who was acting as FI (reval by experience, needed 1 hr with FI) and had to log PUS. However, had I opted for the "flight test" which I didn't need but could have done, and most of the elements would have been the same as the flight I did anyway, I could have logged P1/US.
I agree that P1/US should be the norm for a qualified PPL who is expanding their knowledge by renting an instructor - as long as they are legally entitled to fly on their own.