[quickthrower2]

Fellow Brit here. Born, bred and then f'd off elsewhere. I'd argue P45 is slang for being fired.

Its slang because P45 has nothing directly to do with being fired. It is a tax document related to changing employment.

[lewisflude]

I agree (born and live in London). I've never heard P45 used as slang, maybe you could use "given a P45" to suggest someone was fired though.

[kurtisc]

Something to do with acquiring a P45 as a noun, yes. P45 isn't itself a verb that means firing as I infer the OP to be saying.