Also, this is only mandatory _after_ you've secured a position. I've always concealed past salaries and never had to present my P45 until I've actually started work.
I can only second patio11's point here. My salary expectations have doubled over the past few years by just shopping around and keeping my mouth shut about previous salaries.
But you might not have. You don't need to come up with excuses. Just say what patio11 said. Just say that it's for professional reasons. No point digging a hole for yourself.
It's not an excuse, it's an Appeal To A Higher Authority.
Similarly a hiring manager will tell you that she has to have your salary history and can't possibly go above an arbitrary number because HR and the VP of IT just won't stand for it. Each gambit is equally misdirecting.
It's only an excuse (in the sense meant here) if you actually would disclose other employers' pay policies outside of a salary negotiation. As long as you don't do that, I'd be inclined to call it the truth.
I can only second patio11's point here. My salary expectations have doubled over the past few years by just shopping around and keeping my mouth shut about previous salaries.