I got this once. I was equivocating, and then the guy was like, well, you're gonna give us your P60 when you join anyway, so you might as well tell me.
I was like, okay, fair enough.
I played it completely wrong though. I should have just reframed it and said, well, I paid this, but you're not competing against my current employer, you're competing against Meta, who I'm also interviewing with this week.
Yep, the advice I always give people for this question is to respond:
"My current salary is xyz, but I'm currently interviewing with several companies, so my salary expectations are tied to what I'm worth in the market, not my current pay. My specific ask will change as interviews progress"
i don't know what a P60 is, but you are unlikely required to give them anything except background checks and proof of qualifications until the salary is decided and the contract is signed.
This is very dodgy on their part. Personal tax details should not leak out from the finance department.
In small companies this does mean people have to operate with internal Chinese walls in their heads.