The "only accessible by water" part meant that Britain had a much stronger naval tradition than - say - Germany. That tradition also looked westwards to the relatively open opportunities provided by the Caribbean and eventually Canada and North America, more than it looked towards South America (too far away) and the Mediterranean (already saturated.)

Originally the tradition was based on outright state-sanctioned piracy. But that's incidental.

Establishing a strong Atlantic trade gave Britain the technology, the money, and the economic and military traditions to colonise India and parts of Africa. Trade also drove the Industrial Revolution at home, because there was insatiable demand for goods and they also had to be transported across the country. So: mechanised factories, ships, and railways, and traditions of engineering, research, investment, and military organisation to support them all.

The failures of WWI started to undermine the empire, and WWII all-but killed it. Technological dominance fell with it. The UK was basically bankrupt after WWII and the US took over as a hegemon. After a few humbling experiences - especially Suez - the UK lost its imperial sense of self. Technological decline followed.

The UK's ruling class has never been able to come to terms with loss of empire, which is why we have silly spasms like Brexit - a small country trying to pretend to itself that it's a much bigger country.

Ironically the UK still punches above its weight in a number of areas, including the arts, finance, law, and STEM. But instead of promoting those talents, the ruling class has damaged them even further in search of a fantasy of empire that hasn't been current for over a century.