Too late for it to be any use to this couple unfortunately, but the advice in a case where there is an SDP and the older partner is the lead claimant for working age DWP benefit is to claim UC just before reaching pension age in order to get run-ons of both HB and (in this case) IS.
If they claim UC within a month of becoming a MAC, they should at least have a transitional SDP element included in their UC calculation.
