Counterpoint, I received a tax rebate in the post a couple of years after leaving Belgium to return to the UK. Somehow I doubt the UK's tax office would be so diligent in seeking out former taxpayers abroad to return money to them!
They are. After a 6 months internship in the UK and without telling/asking them anything, they returned money to me a year later.
Can confirm, I also recieved a nice rebate almost a year after leaving my old job due to a clerical error by them meaning I was taxed for a few months after I left.