They were forced to provide slots for up to 3 flights a day to enable another operator to fly LGW/BFS.How those slots were sourced was at the discretion of IAG. They chose to find the slots by cancelling the LGW/BHD route.
If they wanted to continue to op the route at that time they could have sourced the slots by cancelling or reducing frequency on another LGW route.
When Ryanair handed back to slots IAG have had to operate other services to maintain grandfather rights on the slots.Since they haven't reinstated
LGW/BHD they have chosen to use the slots elsewhere.
There is no restriction on them operating to either BFS or BHD if they wanted to.