-A suitable boat, and the gear for it (Merchant on the Thames cost nearly £200,000 all in!)
-A patch or round that isn't already covered. The margins are thin enough without people competing. Failing that, an area such as London where there's enough customers to support more than one.
-A Boatmaster licence (CRT requirement for hazardous cargo)
-to become a Registered Dealer in Controlled Oils and know all the paperwork requirements
Enough savvy to use social media to contact your customers and arrange your deliveries
Places to load (Harder than it seems, Jules on the Southern GU has to run a pair as there aren't enough loading points any more for regular resupply and she needs to carry enough stock.
Either a willingness to cover a large area and work almost every day, or a side interest or job e.g. fendermaking to top up your income
A willingness to get more acquainted with poo pumps and their contents than you thought likely- often just before your lunch
The skills, fitness and willingness to load many tons of coal and gas bottles manually, and heave it up onto cabintops; to fight a loaded boat through shallow cuts, bladefuls and bunged up bridgeholes to get to your customers
The knowledge that you will never make a fortune doing it, it's bloody hard work over long hours for little financial reward.
But then again, not all rewards are financial!
It'd work well on the Thames, the prices are high enough that you can undercut the boatyards. Talking to the operator of Merchant, he sells almost entirely diesel, very little coal and gas.
Or run it with fuel on the motor and a butty with the coal, gas, and pumpout!