The point of entry, and what happened thereafter, is documented in the Grey Friars Chronicle:
"In this year 1348 in Melcombe, in the county of Dorset, a little before the feast of St. John the Baptist [that is, around the 25th June 1348], two ships, one of them from Bristol came alongside. One of the sailors had brought with him from Gascony the seeds of the terrible pestilence and through him the men of that town of Melcombe [see note below] were the first in England to be infected."
Soon people in the villages and hamlets near Weymouth were ill.  When villagers left the area, to seek refuge in other parts of the country, they spread the infection.  It did not take long to reach the major cities.
Because people in the strategically important town of Portland also succombed to the plague, the quarries and fields could not be worked and the coastal defenses were deserted.  In 1352, Edward III ordered that the islanders could not move about as they wished. 
Within a month of its arrival in Dorset, it had spread throughout the county and across to Devon and Somerset, reaching Bristol on August 15th. Citizens of Gloucester, the next large town north refused to allow anyone from Bristol to enter their city, believing that the disease spread on people's breath. This halted the spread of the disease only briefly and it reached Gloucester, Oxford and London by late September. Despite the belief that it was punishment from God for excesses in life, monasteries were affected as much as ordinary villages. As the first epedemic  spread through England, it immediately caused problems in the villages as workers died and food production fell. Farm animals wandered unattended, corn went unharvested and grass grew in the streets of the cities. A horse which previously could have been bought for 6 shillings (30p) now cost forty (£2.00).
In the end, as noted on the plaque shown above, between thirty and fifty percent of Britain's entire population died of Black Death.