As someone who was in the process of booking this trip for Oct 9 , I am puzzled that your sister company Travelsphere can still take bookings whilst Just You cannot get any further rooms to fulfil their trip and my holiday is now in doubt.
I was offered to go with Travlesphere on Oct 4th without a UK tour guide, but preferred to go with JY
So as you are both under the same banner i.e. sister companies and invariably secure block bookings, why can you not just transfer a room booking rather than disappointing a loyal, long standing customer.
I know that my booking is being held in abeyance in the hope that the hotel in question can free up more rooms, but surely this could be a possible solution.