You might be surprised to find that they're still around. Mr. Potter is still alive as well, according to the website:
We have a long and proud heritage, and I’m delighted that the principles of quality, expertise and attention to detail I laid out when I set up shop back in 1812, are still followed by the firm to this day.
That's a very good testament to herbal medications. But what of Clarke? He joined the firm when Henry Potter III was running the shop in 1873. His name was dropped in the 50s, it seems.
The location at the time of this catalog: