I just don't think people need a tour for Borough Market, being quite honest.
It's easy enough to just wander through and look at all the great food stalls.
It's pretty self-explanatory after you know the history is that there's been a market on this site dating back to at least the 12th century. The present buildings were built in the 1850's.
That's basically what the tour guides are charging you for, as well as leading you around to look at the stalls (which you can do yourself).
We spent a lot of time wandering through Borough Market the last two times we were in London. Our favorite things were sampling the cheese stalls, eating a grilled chicken sandwich from one of the vendors, buying goodies to take back to the hotel for snacks, such as freshly made breads, bakery items, cheese, fruit, and wine.
It's easy to just meander around Borough Market on your own:
Here's a page with a map of the food areas:
Note that the church in the photo at the top of the page is Southwark Cathedral. This is easy to locate as you walk along the riverfront on the south bank.
Browse these pages to see the traders and stalls, to make some food choices before you go.
Food tours here for anyone who wants that:
The Totally Tailored looks good:
There are two restaurants where we have had a meal. Fish! Restaurant and Roast which is
upstairs over Borough Market 2nd floor (elevator/lift at the back of the market).
Roast is really expensive but good.
Borough Market is crazy crowded at lunchtime.
Plan to get there slightly before lunchtime if you can.
As Claudia said, since it's been touted in guidebooks and on foodie websites as a "must do" place, it's been elbow-to-elbow crowded.
Another issue is where to sit and eat the food you buy. Most people are standing up, eating, as places to sit are almost nonexistent. You could gather up your goodies and head back to the riverside to look for a bench to sit on while you eat.