And/or

Hi
If the United States and the United Kingdom ____ to do it, ...

Here the bold phrase forms a plural compound subject, thus taking a plural verb, decide.

If the United States or the United Kingdom ____ to do it, ...

Here the bold phrase forms a singular compund subject(use of 'or') thus taking a singular verb 'decides.'

Thank you very much for your reply, Prajwal.

If, however, I do not change my original sentence, I was wondering which form of the verb should go in the blank space.

This has been discussed in a long thread that I can no longer locate. As I recall, 'A and/or B' means 'A and B or A or B'. therefore, the verb number would depend on the number of the nearest noun (B):

If the US and/or the UK decides
If my mother and/or my sisters decide
If my sister and/or my mother decides

Thank you for teaching me the rule, sir.