For that Matter, Come to That

This may be a hard question to answer, but could anybody please help me see why you can replace 'for that matter' in [2] with 'come to that,' and you cannot replace 'for that matter' in [1] with 'come to that'?

[1] My daughter is coming down with the flu. For that matter, most of her friends at the kindergarten are coming down with it too.

[2] Why shouldn’t John, or anybody in this class, for that matter, do this?

Is 'come to that' a British phrase?

Yes, it is English, yet found throughout Anglophony.

"Come to that" is in form subjunctive, being a shortened* form of the older English phrase "an it come to that" (meaning "if it should come to that") ; although it can be used with more force than the subjunctive mood might suggest.

As to style and usage, "come to that" is also slightly more assertive and colourful speech than "for that matter".

*[rather like the old saying "as well you should" being a shortened form of the idiom "it is as well you should", come to think of it !]

By the bye, that division in your second example ["Why shouldn't John--"] is a bit off. The idiom "for that matter" should more naturally come at the beginning or end of the sentence, or after the word "or".

En tout cas, these two idioms are comfortably interchangeable.

Now you can use 'em, come the morrow.

Upon reflection (= come to think of it) in your Example [1] I find that that very phrase "Come to think of it" (or "Now that I think of it") would be a better first clause for the second sentence in the example. The reason I am coming back to say this is that the situation with the daughter and her schoolmates really lacks the futurity or contingency which is preferable for the proper use of "Come to that" or "For that matter".

In other words, these two idioms which you asked about are best applied to something that might be going to happen, whereas the flu in Example [1] has already taken hold of the children. There's no "coming to that", you see - it's come to pass that they are "coming down with it".

So a better way to express the thought behind your Example [1] is to substitute another phrase having a different connotation and say :

My daughter is coming down with the flu. Come to think of it, most of her friends at the kindergarten are coming down with it too.

By contrast, your Example [2] does have a considerable sense of what will or might be possible, even conceptually actionable, in what John or the other pupils in his class might be able to achieve. None the less, I can say that any of the four idioms above would suit Example [2] quite well.

I would be interested to read the interpretations of the other folk who frequent these pages.

"Come to that" (as a phrase unto itself) is not unknown in the US, but I don't think we use it very much. We would be more likely to include it in something like "if it comes to that" than to just say "come to that". However, the meaning isn't quite the same.

We do use other similar phrases like "come to think of it", however.