Yes you would. I live in Australia, having emigrated from the UK. We have booked a Saga ocean cruise in June, since we will be in the UK visiting family.
As others have said, we do not get the benefit of the included insurance as we are non-resident but got a very small credit for that element. We have taken out our own separate insurance. I booked by telephone 😊👍
I still have a UK passport but am using my Australian passport for the cruise because the UK one doesn't have 6 months validity left on it, so I don't think that it would matter if you are not a UK citizen.