Hi,
Article 17 of the UK / Canada tax treaty, coveres pensions and annuities. As the article does not mention pension lump sums in any way, this means that lump sums are not covered by the tax treaty and as they are not covered by article 20A 'other income' either, there is no tax relief available. For this reason, the lump sum is taxable in the UK, as well as in Canada.
Please have a look at article 21(1)c, the elimination of double taxation. This suggests that to prevent double taxation arising, you should claim a tax credit in Canada for the tax paid in the UK.
1978 Canada/UK Double Taxation Convention
Annuities, not purchased as part of a pension plan, are taxed at 10% in the UK under the DTA.
Thank you.