Just had response from P&O:
"Yes, it may be possible to leave your luggage on board if you are disembarking during a world cruise for collection in Southampton. This service is dependent on the ship having sufficient storage and a charge of approximately £35 will be charged to your on board account. Please speak to Reception once you are on-board to enquire about availability. If this service is available, you will be sent details whilst on board detailing what you need to do and when and where your luggage can be collected from, when it returns to Southampton.
The limit is two pieces per cabin you can leave on board (providing there is sufficient storage). This service is subject to change due to legal restrictions. Some ports are very reluctant to allow unattended luggage into their waters."
Not sure what to do now! What if they say no? We'll have to pay excess baggage on our flight home - which can be very expensive. But 20kg each for luggage on flights from a half world tour is not going to cut it. Same posh frock for every formal night??!