[margalabargala]

While you are correct that "eccentricity" as an adjective is a correct usage, OP used it as a singular noun in the preceding sentence, making it ambiguous whether they are appreciating the British for their eccentricities, or for the particular mentioned eccentricity.

If OP's intended meaning matches the adjective as you describe, then it would be clearer communication to use "eccentricity" in this context.

[DiogenesKynikos]

It's not really ambiguous at all, unless one is being pedantic. "I like the Brits because of their eccentricity" is perfectly normal English, referring to their overall quality of being eccentric.

[adammarples]

If you like us for our eccentricity, you'll love us for our pedantry

[whycome]

The sentence before it with “an”

[DiogenesKynikos]

You can technically read it that way, but that's not how I think most native speakers would read it.

[rrook]

> OP used it as a singular noun

Is that the case? Reads as a possessive adjective to me. The OP likes the Brits because of the characteristic that they possesse.

[rrook]

Nevermind, as someone else pointed out, it is used as a noun. Still, it's the correct usage in the possessive context.