BHP, or Brake Horsepower, is how much power is produced at the crank of the engine, before it runs through the transmission and to the wheels. It's called this due to how the power used to be measured, using a 'brake' dyno.
WHP, Wheel Horsepower, is how much power the car actually puts down to the ground. The transmission has frictional losses, so WHP is always less than BHP. AWD cars tend to have more losses than FWD or RWD cars, but a good rule of thumb is about 10-15% loss.
HP by itself could refer to either, but usually refers to BHP if there's no other context clues.
I suppose you use 'tonne' or 'ton' if you live in England or the USA. Here, it's BHP per ton (2000 lbs). You simply find it by dividing the horsepower of a car by how many tons it weighs. Usually, here in the US, you'll see pounds per horsepower, which is just the weight of the car divided by how much horsepower it has.
A Lotus Elise weighs 1980 lbs, and puts out 190 bhp. So, it has 191 hp/ton, or 10.42 hp/lb. F1 cars have ~1250 hp/ton, while a Prius has about 73 hp/ton.