Hi,
The guidance at DT4617 below, advises that, where a UK resident makes a lump sum withdrawal from an Registered Retirement Savings Plan (RRSP) or an Registered Retirement Income Funds (RRIF), Canada imposes a 25 per cent withholding tax.
DT4617 - Double Taxation Relief Manual: Guidance by country: Canada: Withdrawals from Canadian RRSPs/RRIFs
No tax credit relief is allowable, which means that the full lump sum is taxable in both Canada and the UK. You can, however, claim Foreign Tax Credit Relief of up to 100% of the foreign tax deducted, against your UK tax liability.
Thank you.