1875 1876 Alexander Forfar is at 81 Wellington street Luton.
Bedfordshire Electoral Registers.
He doesn't appear later.
There's a few trees on Ancestry with death 1877, but non have full details. Some have him in Ontario

No one has a death for Sarah. It's so frustrating. I suppose a long shot is trying the Bedfordshire Records office with the information you have, you will have to pay for a search.
I had to do that for a marriage in Merthyr in 1880s. It wasn't showing anywhere online but it was in Welsh newspapers
It was found because I had the exact date and place. How it was missed goodness knows.
Who registered Alexanders death, what was the full address. There may be clues.