To convert from a foreign currency to pounds sterling, please use the official rate of exchange at the time of the exchange.
It is up to you whether use the yearly rate, the monthly rate or the spot rate.
If a conversion was undertaken to deposit the money into a UK bank account, then this figure can also be used.
The pension will need to be declared on SA106, including the tax deducted.
Your father can claim a foreign tax credit relief for the foreign tax paid.
The supplementary page SA106 must be completed using pounds sterling only.
The official exchange rates can be found at Exchange rates from HMRC in CSV and XML format - GOV.UK (www.gov.uk), SA106 supplementary page :
SA106 Completion notes :