Therefore I think that sentence above is just a rhetorical question, asking what could be more British.
I'll give you another example to make myself clearer.
'You have such a big mouth, you have let the secret out. This is so you.'
This is so you, means that it happens to you all the time (on the regular basis), and you are well known for this. We all know that you can't keep a secret.
Please note: I am not a teacher.
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