Yes, she brought them for the picnic.

[Queueuey]

Let's look at two dialogues.

1.
A: That's a nice photo, Bill! I can see that your wife is frying potatoes.
B: Yes, she brought them for the picnic.

2.
A: Hello, Bill! Is your wife at the picnic with you?
B: Hello, Alice. Yes, she is.
A: What is she doing?
B: Frying potatoes. She brought them along with other vegetables.

Are these equally idiomatic and grammatical in British English? How would changing the tense to Present Perfect change the style, register, tone, and the listener's perception? Does the choice of tense vary across different social groups? What are the implications behind using either tense in these examples?

 

[jutfrank]

Where did you get these poor dialogues from? How can someone be frying potatoes at a picnic?

The choice of tense is determined by meaning, not by style, register, tone, or social group. Both verbs are in the same past simple tense here, anyway.

 

[Queueuey]

Where did you get these poor dialogues from? How can someone be frying potatoes at a picnic?

The choice of tense is determined by meaning, not by style, register, tone, or social group. Both verbs are in the same past simple tense here, anyway.

I created them as examples.

I asked not so much about the difference between the two Past Simple uses but about the difference between the dialogues with the Past Simple and these:

1b
A: That's a nice photo, Bill! I can see that your wife is frying potatoes.
B: Yes, she has brought them for the picnic.

2b
A: Hello, Bill! Is your wife at the picnic with you?
B: Hello, Alice. Yes, she is.
A: What is she doing?
B: Frying potatoes. She has brought them along with other vegetables.

 

[jutfrank]

I created them as examples.

Please don't create your own examples. Ideally, you need to study real, good examples, with a clear meaning, not artificial ones where you don't even know what you mean.

I think what you're asking is a general question about the meaning and use of the present perfect. We use the present perfect to show that there is in our mind a present relevance to what we're saying. Another way of saying that is that a present perfect sentence is about the present situation rather than the past state or event that led to it.

My partner to me at a picnic last weekend:

Wife: Frank, where are the buns? Don't tell me you forgot the buns?!

My wife used the verb forgot in the past tense because she was thinking about what I failed to do in the past.

Frank: Oh, sh*t. Sorry, love. But forget that—look what I have brought—two bottles of Prosecco!

I used the present tense to show that what's more important is not what I didn't do but what we're going to do now.

 

[Queueuey]

Wife: Frank, where are the buns? Don't tell me you forgot the buns?!

Frank: Oh, sh*t. Sorry, love. But forget that—look what I have brought—two bottles of Prosecco!

What about this:

Wife: Frank, where are the buns? Have you forgotten them?!

Frank: Oh, sh*t. Sorry, love. But forget that—look what I brought—two bottles of Prosecco!

 

[jutfrank]

What about this:

What about it? Why have you swapped my tenses? Please don't do that!

 

[Queueuey]

What about it? Why have you swapped my tenses? Please don't do that!

Why shouldn't I do it?
What's the difference in meaning, idiomaticity, style, register, tone between yours and mine?

 

[jutfrank]

Why shouldn't I do it?

You can't and shouldn't change someone else's words. Especially since I chose those tenses for the specific purpose of giving you examples.

What's the difference in meaning, idiomaticity, style, register, tone between yours and mine?

Yours have the wrong tenses, because you changed the right ones. I'm not sure sure you understand my point: Don't make up your own sentences and don't change other people's words. You need to study real examples of use, or ones that a teacher has prepared for you. Also, forget about idiomaticity, register, and tone. Those are not relevant to what you're trying to learn. What you need to focus on is the meaning (or what we call 'aspect') and use of the present perfect.