I'm in the UK too. When I was hired for my last full time job (about ten years ago, so memories are a little fuzzy) when I was asked what my current salary was, I just replied "I want x", rather than saying what I was earning. They could find that out from my P45, but that was after everything was signed and I'd started at the new place.

At my previous job, I was underpaid compared to the average rate for developers in my area, so I needed to short cut a decent payrise. I wasn't asking for an outrageous amount, but so the company who were hiring me didn't find what I was asking for too high to pay.

I don't know whether they actually ever checked my P45 for my old rate, I doubt it. That side is handled by the accounts/payroll department, rather than upper management. If the upper management were that concerned about the rate at which they hired me, they'd have negotiated down at the time, not once they'd hired me.