Not sure, but Kent and Suffolk were not actual maids.
They appeared in Newcastle's raid (which consisted entirely of royal maids), so is pretty safe to assume that they're considered/regarded as royal maids.
The reason behind their absence is most likely is their hull typing (they're heavy cruisers, while the lasses on this picture are light cruisers...all of them).
Would also like maids with more traditional outfits included (Curacoa and Curlew) Neptune would also be nice (although she's a waitress-cosplaying ojousama)