I don't know the answer for sure and perhaps the film mentioned does tell all but I reckon the fighter boys did most of it by pilotage. Eg follow the river, third airfield on the left. They got used to the major landmarks and probably did some ded reckoning as well. If you had a navigator perhaps a bit more science went into it.
They were doing 200-300kts so you ran out of country pretty fast no matter which direction you chose to go. No matter where you are by my reckoning you will hit the coast in less than an hour.
If they got lost they landed at one of the many airfields scattered across the country asked where they were, refuelled and took off in the right direction, or if the weather was shockers they got a bed in the mess and did it the next day. There are many stories in the books about landing at the wrong place.
The big difference in those days was there was no controlled airspace or air traffic so it didn't matter as much if you were uncertain of your position. I also reckon in real ifr probably a lot of them didn't make it. I wonder what their minima was? You needed good viz to drop bombs so perhaps that precluded flying in terrible weather. They had fido strips for when it was really bad , big wide runways that they burned fuel around the perimeter to burn off the fog.
That's my theory anyway.