How do I get my income tax rebate when leaving the UK?

Calling all UK flutherers…

I am permanently leaving the UK next week. I am currently working full-time, and I used one of the many tax rebate calculators, which says that I am owed about 300–500 pounds since I’m not working until the end of the tax year (April in the UK I believe).

Since I am leaving the country, what process do I need to take to get that money? I am from the US and don’t quite understand UK taxes.

I read somewhere to get a P45 from my employer before I leave? Or does the employer itself process the tax rebate?

So confused!