Straight Away/Immediately/At once

[Take_It_Easy]

Can you explain it to me please what are the slight differences between these expressions:

- Straight Away

- Immediately

- At once

it's more common to use "straight away" here in the UK. - as I've noticed

So is there any slight difference in the meaning of these phrases?

Cheers

[Mister Micawber]

There is no difference in meaning. As you mention, 'straightaway' (one word) is not so often used in AmE.
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Native English teacher at Mister Micawber's