Hi Armstrong35. Condolences to you for the loss of your Pop.
I am not aware of a slow version of A Grand Old Flag, other posters may be. At my father's funeral last year, we played a slow song at the end that reflected our Scottish roots. As thia aong faded, A Grand Old Flag trumpeted through. We had the wake next to the chapel immediately after this. It seemed to work well, honouring Dad's demon love and finishing on a high note.