These things do have to be planned. They can't be put anywhere. There are transmission losses to be taken into account. grid loading, and phase imbalance.
The UK mains supply is 230V AC 3 phase. If the phase loading is not balanced it causes issues at thermal power plants. Because solar produces DC that has to be put through an invertor to convert it to AC, it's phase can be selected, so it is a good way to balance the phase loading.
On top of that, there is a different balance. The grid balance. Because basically, electricity is "sucked" by whatever is running on it. And it takes the path of least resistance. So if an electric arc steel furnace is loaded in Wales for example, it pulls the current in. And that can put a massive load on a local power plant, but might hardly draw anything from a nuke plant in Suffolk. So the grid has to be constantly managed.
Really we need a variety of electricity sources. And this all has to be planned and managed. So when someone says we want a wind farm here, or a solar farm there, it is planned and calculated. And the features of each source taken into account.
If someone wants to build a generating system somewhere. it is the Grid people who will have the final say if the power is wanted or not in that location. So when you say local developers don't know if the grid can handle their electricity, I suspect that means the grid people are saying they might need switch gear hundreds of miles away, or step up step down transformers to get the power from say the Pennines, to that steel works in South Wales. And that will cost money. So it becomes a case of is it cost effective?
It is not the wild west. It all has to be planned and evaluated.