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Would you like to have something to eat. No thank you I------------------(just have) dinner.What would be the correct tense in the above balnk?
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>>Would you like to have something to eat? No, thank you, I just had dinner.

This is - as I was taught - a classical example to show differences between British and American English:

In AE you'd say "I just had dinner.", while
in BE you'd prefer to say "I've just had dinner."
Comments  
Would you like to have something to eat? No, thank you, I just had dinner.
Students: Are you brave enough to let our tutors analyse your pronunciation?
 Pemmican's reply was promoted to an answer.
I have just had dinner.