Sometimes people add the phrase "thank you very much" to the end of sentences when they think the statement is obvious. For example the Dursley's were obviously normal and it didn't need stating, but because it was stated the author added "thank you very much" to show how they might be offended in having to tell you they were normal. "I'm not conceited, thank you very much." "I know how to change a light bulb, thank you very much." It's a little similar to the word "duh". "I know what a stop sign is, duh!" Except "thank you very much" is more polite and less juvenile.

Exactly. It is used here as a form of false politeness, as in the expression of a biting sarcasm. It basically means the equivalent of Shut Up and Go Away.. It means the question or issue is not subject to any questions. Any fact indicated or claimed is final that That---Is----That, Thank you very much. Of course, even in this usage it can be an indication of various degrees of coldness. It can be modest in its expression, as it appears in your book. Or, it can be spoken as a veiled threat, meaning that someone is about to slap your face if you say another word. . I have answered this to the best of my ability, thank you very much. I would write more, but thanks to the need to work so hard, I am now exhausted, thank you very much. See? . .