Double Taxation Canada

I reside in Canada. I am due to receive pension annuities from the UK in 3 months. The amount will fall just under my UK personal allowance. I have found the DT forms online to fill in. My questions are 1. Can I send in the forms before I start receiving my pensions as I understand there is a back log? 2. If not, can I send the form to my Canadian Tax Centre early, and once I receive it back wait to send it in to the UK once I start receiving the pensions. This would only be a delay of a month or so depending on how long the process is. 3. If I send in the DT form and there is a backlog, then the overtaxation that I am claiming back would be an incorrect value, as each month more tax would be taken. How would I claim that back? 4. I have been led to believe by my pension companies that I would be at first taxed under the emergency tax codes and heavily taxed. Would i automatically receive a tax refund at the end of the year? Thank you.

Hi,
The DT individual: Double Taxation: UK/Canada (Form Canada/Individual) asks for the commencement date of the pension.  
This cannot be a future date.  
The completed form is sent to the Canadian Revenue Service for validation.
The CRS will return the validated form to you, so that you can send it to HMRC at the address on the front page.
After allowing sufficient time to pass, for delivery of the validated form, please contact HMRC helpline on +44 13 5535 9022 (from overseas only) or contact our webchat facility at: Contact HMRC.  
We can check for receipt of your form and may be able to process your request.
Thank you.

Thank you for your reply. What confuses me is there is a section for claiming back any overpayment. After filling out the form and sending it to the CRA and then sending it to the UK, this amount will be incorrect as it will be many months, if not a year later. How will I claim back those amounts? And will I need to fill out the same form for pensions that start at a later date? Thank you.

Hi,
The DT individual form should show the date you left the UK, as well as the pension that you receive. The validated form, stamped by the CRS, is our authority to go back to the date the pensions ceased to be taxable and refund any overpaid tax.  
We would also send your pension provider a tax code advising them not to deduct tax. This could be backdated to the start of the tax year, so that they repay the tax for the current tax year. In this way, the current and previous tax years are reviewed and the overpaid tax repaid.  
If you have pensions that will start at some time in the future, you would need to submit another DT individual form then, as you cannot submit the form for future dates or pensions you are not in receipt of.
Thank you.