Not specifically about TPN, but nearly 30 years ago, my dad, who had renal failure and relied on peritoneal dialysis 4 times a day, achieved his wish to go on a cruise- on Canberra. It involved delivery of enough bags of fluid for 4 changes a day plus associated medical sundries. It was all arranged with the cruise line and sufficient bags were delivered straight to the ship. They didn’t need refrigeration. The room steward brought a box when required and all went well.
I suggest you liaise ASAP with P&O medical department as I’m sure it should be possible as TPN at home must be much commoner than it used to be. Your local hospital specialists probably can help with arrangements too.
Insurance should be possible too though may require a specialised company.
I hope you can sort things out and can continue to enjoy holidays ( whether cruises or not). My dad had several holidays abroad, as well as the cruise, which involved delivery of supplies to his holiday accommodation and details of local medical facilities should they be required ( they weren’t!). If it was feasible in the early 90’s I would hope it would be feasible now
best wishes