However, further investigation reveals very bad news from The Telegraph web information site [edited] viz:
'Once you are UK resident, any withdrawal made from the RRSP will be taxable and must be declared in the UK. The 15 per cent [you say 25%] Canadian tax withheld at source would not be available for you to offset against the UK liability. This is because the Canadian tax withheld is on the lump sum withdrawn, but the UK charge arises as a result of the disposal of the underlying assets.
A credit for foreign tax will only be given by the UK if the foreign tax incurred is on the same source, which it is not.'
The only good bit of news is that your fund is a Qualifying Recognised Overseas Pension Scheme (QROPS).
As a 40% taxpayer it looks as though you are going to be hammered again and at least 50% of your Canadian pension post will disappear into the tax man's sticky fingers. I know Benjamin Franklin said that in life there are but two certainties, death and taxes, but this is really nasty. I would be inclined to seek advice in writing from your tax office as to the actual position before making a move in this matter, although I don't hold out much hope of a favourable conclusion. Sorry to be the bearer of such sad tidings. Please don't shoot the messenger!