Sadly, for most of recorded history, Liverpool was but a fishing hamlet up a muddy tidal creek off the Mersey. In fact, it was so insignificant that the Vikings named Litherland and Otterspool to either side, raided Garston, gave Liverpool a miss. Local population centre was 'Walton on the Hill', high on a big sandstone ridge above Liverpool's fishing hamlet. Too far, too steep for Vikings to conveniently raid...
Neolithic people etc seem to have liked their then-wooded ridge, placed some standing stones. Oddly, the Romans don't seem to have bothered with so much as a watch-tower. They certainly preferred Chester, which had a convenient river crossing, convenient river access, was better placed as a road nexus...
Liverpool did later acquire a nice, four-square Medieval castle to control inlet and estuary, but that was 'slighted' during Civil War, subsequently razed...
IIRC, wasn't until the Liverpool creek was enclosed as a dock that business took off. Then, tragically, to eternal shame, Liverpool became one of the 'Triangle Trade' ports, more convenient for sailing ships than working up-Channel to London...
There's more chance of Runcorn / Widnes 'twin towns' going large, as they were better placed for much of history.
IMHO, you need to do something drastic to London, but it would need ASB geology for eg South of England to 'sag' a lot further, a lot sooner after last ice age...than we see