Why would the National Grid limit the amount of fossil fuels we combust at peak times of energy demand?

What do we know about fossil fuels?

We know they have a high energy density upon combustion, hence why we've combusted them since the beginning of the industrial revolution to increase economic growth and quality of life.

But fossil fuels are NON-RENEWABLE resources. Thus they are running out - we cannot consume them forever. We must cut our consumption of fossil fuels to CONSERVE them for when we need them most. In addition to this, upon combustion, they produce pollution and greenhouse gases - CO2 (g) & H2O (vapour) which causes climate change. To LIMIT the severity of these problems, we must use renewable energy resources.

What do we know about renewable energy resources?

Renewable resources don't involve production of greenhouse gases or pollutants so don't impact the environment. The issue is that energy generation depends on the weather, e.g wind farms require wind. In the UK, weather fluctuates over time, so energy production is UNRELIABLE. We also don't currently have the technology to store energy efficiently, making energy production from renewable resources UNFLEXIBLE.  We still need to combust fossil fuels in times of peak demand, where these renewables cannot support this alone.

To summarise, the key points you must consider are:

Fossil fuels are unsustainable, they produce greenhouse gases which causes CLIMATE CHANGE, and are UNRENEWABLE resources. They have a high energy density upon combustion so we must CONSERVE them.

Renewable resources are more environmentally friendly, however they have a low energy density, and depend on weather which may fluctuate - making them UNRELIABLE, and MAY NOT SUPPORT ENERGY DEMAND ALONE. Therefore we must produce energy during peak times with renewables, for sure, and by combusting SOME fossil fuels, but in a smaller quantity, to produce the remainder of energy that's required.