What is the reason for the decline in the rate of economic development in England?

The pace of economic development can fall when a certain growth milestone is reached, for example, at the initial stage of an industrial revolution it is high, and at the final stage it already becomes lower, since there are fewer free niches in the economy.

Secondly, various negative phenomena can occur:

Changing demographic situation, aging population, rising labor costs.
Driving out of the sales markets by competitors, for example, at the beginning of the 20th century, England lost its position of the world leader in the US economy.