Using FX from prior lump sum remittance for subsequent pension payments

When I first transferred residence to the UK I transferred my savings using FX at the time but that was just before Brexit and the pound was much stronger than now. I have since lived on those savings and have not been remitting my foreign pesion and so the FX applicable at the time of pension payment was notional since no money was remitted. My question is whether I could have applied the FX at the time I transferred my savings to my subsequent pension conversion to sterling until such time that the pensions caught up with my savings transfer. The second part of my question is, since in fact I have been using the FX applicable at the time of pension payments, whether I can claim a tax refund or resbmit my tax return and for how many years retrospective.

Hi

For converting to pounds sterling, you would need to use a just and reasonable exchange rate in use at the time of the conversion.

Thank you