I believe the slot question to be a non-issue.
EU airports and airlines operate at less than 10% capacity. There has been a surge of holiday traffic in July/August, but with covid resurgence in Europe and the end of demand for the holiday season, the outlook for September and the next few months is bleak, to say the least.
There is little doubt that LHR/LGW will operate under full capacity for one, two or three years to come.
There is no way that an airline, even BA, will fly empty planes 80% of days, just to keep a slot.
A new system might be needed to replace the 80/20 rule.
But even with that system, it will not be terribly damaging to lose a slot, as there will be a huge number of free slots available in case resumption of a service is desired.
BTW: the Wizz Air case is of little significance. WIzz is a Hungarian LCC. It has a base in the UK, but that is small.