Just posted this across on the MME thread, but exactly the same applies for IOM-LHR.
It was always highly unlikely that anyone was going to get LHR slots for Summer 2021 right from the off.
For Winter 2020, those airlines like Rwandair, Ukraine, China Airlines and Eastern who secured slots have been given these without grandfather rights attached. As the "use it or lose it" rule has been suspended through a slot waiver, airlines at Heathrow can hand back their slots but still protect their long-term access to them. Those slots are then being handed out on a short-term basis.
For Summer 2021, it is not clear whether there will be any change to the use it or lose it rule. Every airline has reclaimed the slots to which they hold grandfather rights and so Heathrow (as of now) is technically full for Summer 2021. Of course, there will be a lot of airlines either hoping for a slot waiver or going into deals with other airlines to babysit slots for them. None of those will be visible at this point in time.
What you see now and where things end up for Summer 2021 LHR slots will be two very different pictures.