Recently with the Battle over Malta or the last few attacks on British Homeland (that I remember from the regular episodes), it seems that the RAF always imparted heavy casualties on the Luftwaffe. Though during the RAF raid on Köln Britain lost only very few bombers.
Welcome. The answer is different for fighters and bombers.
For fighters, the RAF had a very well developed ground-control system with radars that directed pilots to the fight. Pilots were required to follow these intercept orders and not go hunting on their whims. This proved to be very efficient and resilient.
Germany is just starting its air defence system. Watch this space, but British casualties will mount, quickly.