My understanding is it’s not about the onshore/offshore it’s about if it is classed as income earned whilst resident or non resident. The provident fund is possibly not considered as yours until it is released to your personal account for tax purposes. Thus they could say you earnt it when you were resident if it arrives in your account after you arrive.
The trouble staying outside the U.K. is that with Covid it is pretty hard to stay anywhere other than your country of residence