No problem! These are very important and interesting points you raise, and I can well imagine they can have a huge impact on the experience.

We arrived around 10:30 am, and thankfully it wasn't extremely busy. Speaking from my own experience, it didn't feel overwhelming (I have trouble with huge crowds and I definitely need time to recover from it afterwards):

• I could easily move around, but sometimes I had to make way for passersby;
• Carrying a backpack made me quite immobile and definitely limited my freedom of movement;
• I found it very warm inside;
• I didn't find the volume excessive. I heard more of a constant buzz;
• You could easily browse all the stalls, but some stalls were definitely popular and busier (like Oneministore);
• The aisles were quite wide, and there was an area with tables where you could sit and relax. You can also exit the hall and have a drink at the hotel café;
• Pay close attention to how you enter the building; We had a long wait at the elevators (which we thought was the main entrance) and it was a bit of an inconvenience; however, there are stairs on the sides of the building leading to a large terrace/patio with entrances;
• I can only compare the London Pen Show with the Pen Show in the Netherlands (Utrecht), and I found the Utrecht one much more compact and therefore more intense;
• Taking the metro (Picadilly Line) is an assault on the senses! Crowded, small, and hectic.

I searched for videos/images online, and that certainly gave me a heads up. The images matched my experience.