Is a Greek private pension taxed fully for a UK tax resident

Hi If a UK resident - higher tax payer - receives a lump sum pension payment (one off) from a Greek private pension scheme, does he pay income tax on the full amount as...UK income/pension? Is there any change that he will get any sort of relief? He has already paid tax in Greece for this amount. Finally, can he claim that 25% can be taken as tax-free cash ? Thank you

Hi,
You can see guidance here:
Tax on foreign income
Thank you.