What if the Rolls-Royce Merlin engine was never invented?
I'm not sure what the POD would be for this - car accidents that remove some of the design team are possible - or perhaps early problems with the engine convince RR that it's not worth fixing. Or maybe a less solvent RR doesn't have the money for the initial development, as the Merlin was a private venture. Or early theoretical work convinces the designers that the Merlin can't reach the potential of the Vulture and Peregrine. Maybe a mix of several of these.
Were there any serious competitors available? I can't think of any (though I could well be wrong on that) and the RAF did use a lot of Merlins.
From what I've read the Peregrine was simply too small and under-powered.
Although radials could have been used, the drag issues could have saddled the RAF with slow aircraft for 1939-40. Though I assume someone would have invented the larger streamlined spinner that the FW190 used.
Another engine would have to be found for the Hurricane, Spitfire, Mosquito, Lancaster and Mustang. Would these aircraft have been able to reach their OTL performance with the alternative engines?
I'm not suggesting that alt-engined Spitfires would allow a successful Sealion!