UK Immigration Law
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If you want to be absolutely certain of having no issues then I recommend that you get a sworn affidavit from your husband (a sworn oath) to confirm that you have permission to take the child overseas. Also take the child's birth certificate.
The chances are that you will not have any problems - some countries are hotter about children travelling with only one parent than others. Going between the US and the UK it is likely that it will not be an issue and that even a letter of permission would be more than adequate.