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.
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.
That seems to be “eccentric” not “eccentricity” (your failure to cite the source of the quoted definition makes it harder to tell, but it is almost verbatim the M-W definition of “eccentric”.)
(Eccentricity, the noun used, is just fine with “the British” if onenis referring to the collective manner in which they are eccentric, the plural “eccentricities” would be appropriate if one was referring to the diversity of particular manners in which they were eccentric.)
If OP's intended meaning matches the adjective as you describe, then it would be clearer communication to use "eccentricity" in this context.