I'm interested in your post and while I have no experience in the financial sector I did a quick Google search and one if the first hits I got was Ulster Bank. They have a list of approved payments that can be counted towards mortgage and both pension and invalidity pension are included. I guess all banks would have roughly the same conditions.
If you think about it they are both guaranteed payments for the long term, the only issue you may run into is the life insurance part depending on your condition.
Best of luck with your switch and retirement. I'd be grateful to hear how the application for early retirement went if you were willing to do a post on it as I'm in a similar situation to you.