Just because most Londoners don't care doesn't make them unimportant. They can provide a useful 'infrastructure' role for allowing this specific sector to run more smoothly.
There's a similar thing in Toronto, the Ontario Food Terminal, that allows various local businesses (restaurants, local grocers / veg stands, (neighbourhood) florists, etc) buy directly from producers without having to go through a middle-man:
* https://en.wikipedia.org/wiki/Ontario_Food_Terminal
There was some worry that the facility would be retired and the land (e.g.) used for housing, but there are no plans to get rid of or relocate it:
* https://www.cbc.ca/news/canada/toronto/ontario-food-terminal...
* https://farmtario.com/news/ontario-food-terminal-distributes...
Given that Toronto is the largest city in Canada, it allows for a somewhat convenient location for producers to sell to a large amount of buyers in a concentrated space.
> […] and require major financial support.
Running a modern society requires major financial support. The question is: what are the benefits to society of the infrastructure in question? Are the vendors not charged rent? Are the buyers perhaps not charged a membership fee? At least when it comes to OpEx do annual fees not cover expenses? The above mentioned Ontario Food Terminal is self-sufficient from fees (perhaps the government helps out every so often with CapEx?).