First of all, an "impression" implies willfully changing your accent, which is absurd.

If Britain was a Chinese colony where everybody learned Mandarin as a second language, and that country then no longer belonged to China, and in the meantime Cantonese overtook what the whole world basically would come to associate with "Chinese" and you heard that person speak, would you say they were "imitating" Mandarin Chinese? That would be a ridiculous statement, they would be speaking a variant of Chinese fully explainable by their historical context, and assuming they would speak Cantonese because it at that point became the "standard" would be equally ridiculous. There's also no linguistic basis for calling this hypothetical "English Mandarin" fake, an impression, or in any way hierarchically categorize is as inferior to "regular" Mandarin, or as you describe it, not being a "genuine accent" (which is a phrase that is a complete crock in this context).

I'm done with this discussion. Implying Hong Kongers "imitate" British English is nothing short offensive and has no linguistic foundation.