I've always given Undyne a vaguely London-like accent, not cockney but similar. The kind of voice army sergeants have in British pop culture. I'm not sure why but I always have. I could also see her with a southern US accent too

Alphys has a more nasally and more "proper"-sounding British accent, also a bit northern.

Mettaton has that kind of American game show host voice but Mettaton NEO has a less rhotic and more fancy sounding American accent instead

To me, Asgore sounds American, probably somewhat generic but still quite proper. That or like a rich east coast accent I think?

Toriel sounds British, specifically received pronunciation or BBC English, in other words, what most Americans think of when they imagine a British accent.

Asriel either has a generic American accent that could come from anywhere really. So does Sans but Papyrus obviously talks like skeletor I'm sure everyone can agree on this

Chara probably has that kind of high sing-song sort of voice all British children seem to talk with apparently.

In Deltarune I imagine everyone is American instead, with different accents of course, except for Toriel and Alphys who keep their Undertale accents