CZWu

Hi, before I migrated into the UK (non UK tax resident), my previous overseas employer owed me the commission. After I became the UK tax resident, they repaid me the commission. The commission was generated from my previous overseas employment before I migrated into the UK. Do I need to pay the UK tax and/or submit a self assessment tax return on this money? If yes, what the exchange rate I need to use on the tax return because this money hasn't brought to the UK? Thank you.