We just returned from London, and spent last Saturday morning and lunchtime working our way through Borough Market. It was elbow-to-elbow, all the way through. Jen, glad to hear you will be there on a Wednesday! Maybe the crowd will not be as bad as the Saturday we went!
I will caution you about the restaurant, "Roast" which sits above Borough Market. Second floor; take their elevator to get there if you must go. Roast is very expensive. About $120 (in U.S. dollars) or more per couple to eat there. Sixty pounds per person, more or less. I would skip it. We ate there, and I would describe the food as average....nothing to write home about.
If you want to have a pork belly sandwich from Roast, as you said in your original post, there is a "Roast To Go" window right below Roast, just to the left of their elevator. I imagine you could order and get it there take-away. But I can't imagine why you would want it, once you see the other tasty items available at Borough Market stalls!
Of the food stalls below, don't forget to visit Borough Wines, right below the entrance to "Roast". Terrific selection.
The cheese sandwiches at the cheesemaker's stall looked fantastic! That is what I would have gotten if I had not eaten at Roast restaurant. Also, there are numerous bakery stalls around Borough Market...all with dozens of wonderful treats.
About the seating situation....we walked across London Bridge, then down Southwark Street, turned right, walked behind Southwark Cathedral. There are some wooden picnic tables there on the right, as you get to the first two dozen vendors (these had chicken burgers, hamburgers and chicken kabobs). If you are traveling with a group of people, say four friends, I would advise you to grab seating at one of these picnic tables. Leave two of you seated at the table to hold the table. Send the other two people out for food. That's pretty much your only hope of seating. Many people were eating standing up. Some were sitting on the curb.
Farther along the row of vendors, turn left, and walk under girders into the main tunnel-like area of the market. You will find a lot of venders in this area with fresh fruits and vegetables. Keep walking farther, and you will eventually come to Monmouth Coffee Co., which you had mentioned. By the time you get to Monmouth Coffee, you are more or less out of the most crowded part.
If you stand in front of Monmouth Coffee, and look up, you are looking at the windows to one end of Roast restaurant. Monmouth Coffee has its own selection of great pastries. So if you've already had your main lunch at Borough Market, consider coffee and a dessert and sit at one of Monmouth's indoor tables.
Monmouth Coffee is on Stoney Street, so if you come out its front door, take a left, walk north up Stoney Street. That will take you back to the south bank of the river, and out of the Borough Market.
Again, here's hoping it won't be all that crowded if you visit on a Wednesday!