My proposal on the "windfall tax" would be:
Energy giants have their profits capped at FY21 levels (already inflated due to Covid)
Remaining UK profits held by UK government, these profits can be returned to the company based on a sound investment plan to green energy once the investment has occurred and green efficiencies in place (current profit levels should allow for investments to take place). Any profits not planned to be re-invested into the green energy are used to offset the borrowing used to implement the new cap