There are a plethora of unknowns that make it very difficult to check transmission rate.
Transmission is rare, but this could be impacted by lots of factors. There were hundreds of thousands of cows in the UK, but there were also testing and culling programs to try to prevent them from entering the food supply. There were also regulations on which parts of the cows could be used for human consumption.
Last but not least, there are huge uncertainties around the diagnosis rate of mad cow in humans. For example, a 2013 study (1) found that 1 in 2000 people in the UK now have Creutzfeldt-Jakob disease.
Based on this, it could be possible that transmission rates are very high if a human were to eat mad cow nervous tissue.