Yuki, I haven't left the UK, but I've helped two friends leave it, so here's the list we built together, in rough order.
- the workplace pension usually stays where it is, and you keep the right to it. Get the latest statement, update your address, and make sure the provider has an email that will still work after you leave
- tell the tax authority you're leaving, because it affects how the final year is taxed, and sometimes you get a refund
- keep one UK bank account open if you can, at least for the first year, for refunds, the last salary and anything that arrives late
- close the phone contract last, or move to a cheap plan, because banks and the tax office will still text you
- council tax, utilities and the rental deposit need final readings and a forwarding address
The thing both friends wished they'd done earlier: collecting every document about their time in the UK, payslips, tax statements and pension letters, in one digital folder before the last month, when everything becomes chaos.