Begin at the beginning;
British television was to educate and inform. Beginning in 1936 it was free of advertising. Money was made by subscription services.
American television began in 1939 with the broadcast of the Opening of the World's Fair and a speech by Franklyn Delano Roosevelt, the President.
Ten years later commercial television began with sponsored programming. Getting money from advertisers was the path the United States took. It seemed to be successful as by the 1950s, most American homes had television sets. There were many comedies and dramas created for television.
Using television to broadcast 'news' became an Orwellian 'Big Brother' where people were told what the 'Establishment' wanted them to know. Public opinion was shaped and controlled by these 'News' programs.
The idea of 'half hour' and 'hour' long broadcasts, broken by commercials, (which would drop the actual air time by about eight to nine minutes for half hours to forty two to forty three minutes for the hour long program), became 'standard'.
'Specials' whether sports, awards, special dramas, could last longer, but basically the 1/2 hour / 1 hour pattern was set over sixty years ago in America.
Running 24/7 creating children's programs and running old movies during the after midnight before six a.m. block was also standard.
British programming was not so constrained. Without need to break for commercials, programs could be of any length, and did not need to fill the airwaves after midnight or before 8 am..
This pattern has somewhat persisted, so that today, many programs for English television maybe be 90 minutes long.
Obviously this has an effect on the contents of the program.
As an example, compare an American drama of 'one hour' (43 minutes) with an English series of Ninety minutes.
AMERICAN STYLE
A program often begins with fast paced music, three second cuts of places/people to announce where it is taking place . Within 30 seconds one is fully aware of where when and what; unless the program begins in the middle of an argument/ discussion among the characters.
If the first choice is selected, within thirty seconds you know that this is a futuristic show taking place in space, or a Western taking place before 1890, or today's big city. The Stars of the show may be shown and the actor's name revealed, along with 'give aways', i.e. crime, medical, romance, etc.
If the second choice is selected, one is flung into the action so that one knows what kind of show is being aired, the personalities of the Stars, and what the plot is or going to be.
There is a lot of action, often violent, quick dialogue, and between minute one and minute 43 the issue is resolved.. unless it is a two part episode or part of a story arc.
BRITISH STYLE
A program may begin with an incident. Time is taken, there is no rush. For example, if the scene opens with a car that has broken down, it is not just one turn of the key and the car doesn't start. It will be a turn, opening of a hood, looking around, trying again, so that at least one full minute, if not two is spent to confirm the car is not starting.
Violence is rarely shown. Implied, but rarely shown. Dialogue is measured, slow enough to prove someone is thinking before answering.
Views of scenary, of premises are slow enough so that if one blinks they don't miss it. Time is taken to establish the characters. The issue may take the full ninety minutes to be resolved in clear steps.
Contrast
In America programming it seems getting from start to finish is the goal. Move as fast as possible to 'get it over with'. British television is more focused on the essence. The establishment of the characters, the facts, the events, the interactions. Three seconds is not enough.
Just as a person would prepare to attend the theatre, take their seats, and focus on what is happening on the stage, so too British television.
American television is made up of quick sound/sight bytes moving as fast as possible, cutting out the pauses, the thought, the precursors to get to the 'meat'.
To Watch
If one intends to watch an American program online, it is easy to do a marathon of three or more espisodes, back to back, stopping them to eat or do some household chore. Nothing is missed, really, as the denouement is the purpose.
Most characters are reduced to an 'essence'. It may take many episodes to see the complexity, (if a character is complex, which is rare, as most are 'vehicles' to move the plot forward).
If one intends to watch a British presentation, one is in for the long haul. The show proceeds without interruption, (as there would be no commercials) and one gets more than a 'sense' of who the characters are, one knows them were one reading a novel.
This is the basic difference; British Television serials seem to be novels brought to screen. American programs could never work as novels unless they were taken away by an 'outsider' and rewritten, adding all the nuances left out by the screen writer.
THE PRESSURE OF TIME
As is obvious, an American program needs to have sponsors. It needs to gain its funding from an outside source. It must create episodes around which the sponsors can place their advertisements. If advertisers don't like the program, it goes off the air.
For example, in 1966 Star Trek was aired. It did not get the sponsorship, so went off the air after 3 seasons. It was a 'failure'. Yet, Star Trek is alive today, in movies and in sequels and prequels on Television.
How did it happen that a mega-hit was not appreciated?
British programs do not need sponsors. The producers do not need to please anyone. They can focus on quality and then put the item forward. It may need to be rewritten or redone to please those who will fund it, but do not need to chop and fix so as to make space for ads to interrupt the action.
Watching a British production, the lack of 'haste' becomes apparent. There is no need to hurry a scene, jumping from words to passion, so as to conclude a relationship, time may be taken to develope a relationship.