In many worlds you just wouldn't be able to play a direct equivalent. Magic makes it too easy to stop the fires. In a city the size of Waterdeep/London, it seems unlikely that there would not be spellcasters capable of stopping the fire.
On the other hand, if you make it magical fire, or otherwise a magical attack, then you can do more. However, with magic involved, the fire might have a tendency to work faster.
In my homebrew world, which I've rebooted 3 times (4 total continuities), the campaign begins in a land that is about to be overrun by a surprise attack by a fanatical assembly of forces. One of the things that these forces know they need to do is to quickly eliminate as much of the magic users from the nation to be concquered as possible. They achieve this by creating a mystery that draws many of them to the Tower of High Magic, the governing body of the Arcane in the nation, and then using a powerful magic to rip a hole in the universe between the Prime Material Plane and (my equivalent of) the Elemental Plane of Fire - and then releasing several broad anti-magic spells to prevent fire fighting, teleportation, etc... The spellcasters are trapped within the tower as it is consumed. The rift to the fire plane is in the only 'mundane' entrance to the massive tower (there are no windows, balconies, or room entrances). An army of fire elementals, salamanders, and azer wrap up things up for the conquering army. As the massive tower collapses, it falls on parts of the surrounding city and the city itself is then destroyed in the fire. As it is aided by the fire plane residents, the entire town is consumed in less than an hour.
In all 4 continuities, the PCs were not near the tower when this went down (although in all 4 they actually play an unwitting role in setting it up to happen). In the current run of the campaign, they have not even heard of the events transpiring as the leaders of the vanquished refugee nation are keeping many of the details of the downfall from the populace in general and there were very few survivors from that location.
If you want the feel of the Fire, but are not tied to an actual fire as the mechanic, you could have the city be built upon a Primodial's resting place and then have it awaken, essentially having a slow acting volcano consume the entire city. That type of adventure would be appropriate for PCs of any level. It would just change whether it is a 'avoid everything' adventure, or 'fight the thing before it entirely consumes the city' adventure. I did something like it using Cloverfield as a model for the adventure. The PCs were hired by a very wealthy NPC to recover the components of an artifact that he'd kept locked away in 7 different vaults within the city, so the PCs had to try to recover them before they sunk into lava in their various locations. They didn't have time for multiple short rests, much less a long rest, and had to deal with a collapsing city, fire monsters in the service of the primordial, and eventually with the emergence of the primordial himself (300 feet tall lava monster that was arising institnctually because other primordial creatures were arising and they battle when that happens).
www.historic-uk.com
