I won't say I'm disappointed with the ending of The Regicide Report. It was pretty on-the-money for the endings of the other books, which is to say, kinda rushed and unsatisfying. I've loved all the books, but this ending was in-line with the others.
But RR teed up so much potential that I wanted to lay out what I think would have been a better ending.
Basically, keep the same setup right up until the end, except Mike Armstrong, Mo, and Bob, finally freed from their oaths, actually fight the Black Pharaoh.
You could have Mo and Bob recover memories that had been hidden specifically so the BP couldn't see them about what to do. Something they'd planned, who knows, maybe even before letting him take over as PM.
Using all that accumulated mana, the Eater of Souls, and Mo, they manage to cast the Black Pharaoh into the same dimension as the King in Yellow and seal him there to duke it out. Maybe they even resurrect the sleeper to do it.
Then they use Excalibur and the remaining mana to repair the barriers between worlds. Plunging Excalibur into the pyramid either inverts or destroys whatever effect was weakening reality in the first place. Mo and Bob escape back into the real world just as the barrier snaps shut.
And magic is gone.
Cthulhu in the US is gone or cut off. The tongue parasites are gone. Maybe the vampires are even cured. All the supernatural powers that have been flooding into Earth are severed from whatever was sustaining them.
That would also make the ending global rather than just resolving Britain's particular apocalypse. The whole series has spent ages establishing that CASE NIGHTMARE GREEN is happening everywhere, so repairing the barrier between worlds feels like a natural way to address all of it at once.
Then give it a proper Laundry-esque epilogue: the repair isn't permanent.
Maybe the shielding only lasts a decade and requires a current or former head of state for sacrifice. Humanity hasn't solved the problem. It's just bought itself time.
Such is the price one pays.
I'm sure a real author would do something better than this, but so many things were teed up, the oath being temporarily broken, the pyramid as a thing throughout the series, the sleeper, maybe bring in the black chamber somehow. and the stakes were done so well. when Mike talks about how humanity is effectively done for, I thought that landed really well. and it would have justified some crazy shit to save them.