I think, in formal usage, you will find that American and British are basically identical. We each use both of those constructions in the appropriate situation. Obviously, there is a semantic difference between these two constructions and neither dialect exclusively uses one or the other.
I am not certain, but I think what you are referring to is the fact that American English speakers can sometimes use simple past in places where one normally uses present perfect. So, as a US English speaker, I would correctly say:
(1) I've never gone to a tennis match before, but I am going to one today.
But, sometimes I say:
(2) I never went to a tennis match before, but I am going to one today.
I would not say that this second example is standard US English — in any formal situation I would use the present perfect. But, I suspect it is common in speech and I do it quite often.
This could be the very beginning of a semantic shift in the present perfect construction in English. Perhaps (2) will be preferred in several hundred years. (Such things are not unheard of; German now uses the present perfect form to indicate simple past in speech.)
If a non-native speaker asked me about this, I would never recommend to use the construction in (2), because (1) is right in every situation and never sounds strange or formal.
When 'recently' or 'lately' is used with present perfect progressive:
Since the adverbs recently and lately both suggest that something is done either "at a recent time" or "not long ago", using these words to describe a particular ongoing action in the past does not necessarily imply that the action is continuing right up to the present.
Scenario 1: A is having lunch with B.
A: Have you been listening to the podcasts by Brain Cox recently/lately?
B: Yes. (But this does not imply that the action is continuing right up to the present.)
When 'recently' or 'lately' is not used with present perfect progressive:
Omissions of adverbs such as recently and lately do not necessarily suggest that an action is complete.
Scenario 2: A saw B sitting on a bench.
A: How long have you been sitting on the bench?
B: A couple of minutes.
What about the present perfect?
The present perfect is normally used to denote the present state of an action's being completed, that is, that the action took place before the present time.
Habitual action:
The present perfect and the present perfect progressive can both be used to talk about an action that is habitual:
"I have gone out with her every weekend."
"I have been going out with her every weekend."
Using the present perfect progressive can imply that this habitual action is continuing right up to the present, but it may not always be the case.
Updates: When 'recently' or 'lately' is used with present perfect progressive
Describing an action in the present perfect progressive form with "recently" or "lately" does not necessarily imply that the action is continuing right up to the present, but it may suggest that it is a habitual action.
"I've been going to the pub recently."
Describing an action in the present perfect form does not suggest that it is a habitual action.
"I've gone to the pub recently."
Best Answer
Yes, you finished it. Otherwise, as you say, you would say something like "I started to write my article yesterday". In that case, 'started to write' has finished even if the article hasn't.
No, it doesn't. Present Perfect is 'until now' or 'relevant now'. It says nothing about what happens after now, though context might. For example:
"I have worked in that factory" implies (by using 'that') that you no longer work there. Maybe somebody is asking you what is inside, and you have relevant knowledge because you have worked inside the building.
"I have worked in this factory for ten years", but this is your last day. You have worked here until now.
"I have worked in this factory for ten years", so you know where the toilets are and don't need telling. Unless you get sacked you will continue to work here.
If the action is incomplete, you use present continuous. Past continuous is for a finished action.
In your example “I was watching this film”, the action - the watching - is complete. You are not watching it any more. Whether you completed the film or not doesn't really come into it, as the film is not the action.