"England's Capital" is in reference to the city of London in "Beachy Head" by Charlotte Smith.  London is the center for all things economic, cultural, etc. in England, and is a major metropolitan city. It is also very politically active, containing locations such as the Palace of Westminster and Buckingham Palace. Geographically, London is situated by the River Thames and is directionally in southeast England. It's bustling and busy nature is described in "Beachy Head" as it references it's "mart", which is a trade center.

"But if the eye could reach so far, the mart

Of England's capital, its domes and spires