Earlier quoted context omitted.
No, just the particular fads, e.g., the one that drove the change to the non-rhoticity of RP, didn't exist in 15th century Britain. That the current accent is driven by a particular fad aimed at highlighting upper class status doesn't imply that class distinctions are novel, just the particular effort to demarcate them (which wasn't durable, obviously, as RP isn't exclusive to the upper classes.)
Sure, accents are partly markers of social status, but it's quite wrong to ascribe the diversity of accents in Britain to this. There was never a period where everyone in Britain spoke with the same accent.
No one, AFAICT, did that. The reference to “modern British English” is not, as I read it, a reference to “the current diversity of British accents”, but rather “the current dominant British accent”.