At what point were the numbers of slaves transported by the British to America first recorded? How many people were transported as slaves from Africa to America by the British? How many of them died on the way?
At what point were the numbers of slaves transported by the British to America first recorded?
The numbers of slaves were, of course, recorded to various degrees since the beginning of the trade, which was coincident with the establishment of the colonies. For example, the Dutch are recorded as bringing slaves to Jamestown in 1619 at the very dawn of the British colonization.
The first attempt to comprehensively measure the British trade, including the slave trade, was by Malachy Postelthwayt in his various works, including the Commercial Encyclopedia. His work, "The African Trade the great Pillar and Support of the British Plantation Trade in America" (1745) is the first work by him on this subject.
How many people were transported as slaves from Africa to America by the British?
It is not easy to compute this number because there were no official statistics, each state being an independent entity. According to the Encyclopedia Britannica, the total number imported to both the West Indies and to America was 2,130,000 between 1680 to 1786. This number is probably the estimate of Bancroft. The fraction delivered to the American colonies is unknown. Note that estimates are often exaggerated for political reasons. Carey (1872) made some of the most detailed estimates and he is cited by the US Census as an authoritative source. His estimate was about 250,000 imported up to 1790. This estimate may be too conservative, and the actual figure might be double that. Note that Carey's estimate is for slaves imported by anyone, not just British merchants.
How many of them died on the way?
Bancroft estimated it as one eighth (12.5%) and he is widely quoted, but it is quite a guess. Norris in 1790 stated that 5% of the British slaves died in transit as of that time:
Of the forty thousand purchased by the British, about a twentieth part are calculated to die on the voyage; and three-fourths of those that arrive in the West Indies, are disposed of to the French and Spaniards; the remainder are distributed in the British islands....
A Short Account of the African Slave Trade by Robert Norris (1790)
Conditions were a lot better in 1790 than in 1650 so over time you can assume the percentage lost in transit decreased.