I think the most important reason for the decline of use of coal in the UK was the This is because

The most important reason for the decline in the use of coal in the UK was pollution.

Explanation:

The UK was the first country in the world to industrialize. Its industry was based on the power produced by coal. Coal is a fossil fuel and it is highly efficient when it comes to the production of energy, and with the UK having an abundance of this resource it seemed that it is a perfect match.

The coal indeed helped the industry in the UK to skyrocket, but as time was passing it was also bringing in more and more problems. The major problem was the heavy pollution, which in turn resulted in other problems. The pollution contributed to low visibility, hard breathing, significant worsening of the health of the people, acid rains that were destroying the vegetation and caused heavy erosion. Seeing the consequences, the country moved in another direction and gradually abandoned the coal and replaced it with oil, hydropower, nuclear power, and in more recent times with renewable energy sources.

associative property of addition