Why couldn't P&O offer the staff they just sacked the lower wage package they just gave to all the new cheaper paid staff they hired. Not all would have taken the pay cut, but im sure some would have.
P&O claims that they've been trying to negotiate with the Unions for the last two years without any progress / movement. The company is making losses of £100million per year, I guess this is the result of stubborn Unions and unprofitable companies. If I recall correctly, P&O said that the workers could apply for their old jobs with the new contractor / supplier.
I suspect this was the only way that P&O could reduce their operating costs, however it is a crappy move on their part. Although the alternative is that the company folds.