BTCC generally has more overtaking, but its kinda due to the rules
In BTCC, if a car has a knock, teams have all the time between races to try and sort the car, however, in WTCC, If a car gets a knock in Race 1, teams only have the 30 minutes before Race 2 to sort the car, if they work on it sooner, they can suffer a grid penalty, so drivers tend to go a bit careful in race 1, but hell for leather in race 2
Hope that makes sense