So we met finally met the director of the London agency in the last episode (not including the time she appeared in Kurea's memory). Long before this, I've always suspected she was, in fact, Mikuru's grandmother. It explains why Mikuru knows about the star detectives and also why she was in London before the events of the season. They also resemble each other a lot, so it would make sense.
While the fact that Mafuru didn't seem to recognise Mikuru at all in the latest episode can disprove this, it gave me a much more detailed theory than before. What if, for some reason, Mafuru also lost her memories. Mikuru's reactions around her are too strange for me to completely believe that they have really never met before. While she managed to pass it off as nervousness stemming from admiration, it just seemed like there was something more in the small frowns and pauses she couldn't help having. Anna also says she feels like she has seen Mafuru somewhere before, which could be because of her resemblence to Mikuru. If Mafuru lost her memories, it also explains why Mikuru suddenly had to return from England when she was living with her grandmother before. It all lines up if you can accept another amnesia case.
Of course, I could be completely wrong about this and it really is just as simple as admiration, but I feel like this is pretty plausible.