HB or UC with 1 pensioner & 1 working age

Hi All,
Just hoping for some guidance here.
Claimants are a couple, 1 is pension age receiving SRP and one is working age receiving SSP.
LA are refusing to accept HB claim stating that claimants must claim UC instead - guidance in Shelter (Help with Housing Costs Volume 2 18/19, Table 2.2) states that ‘if you are a couple and only one of you has reached SPC age, you can choose to claim either:
HB (with or without SPC); OR
UC
Is this affected by the fact that one of the claimant’s is receiving SSP and is therefore pulled down in to the work-age ‘pool’?
Any direction to legislation would be much appreciated!
Thanks in advance

This keeps popping up, see this thread which should help https://www.rightsnet.org.uk/forums/viewthread/12391

Much appreciated, will take a look through.