Confusing scenarios like you mentioned are why highway code rule 110 says you should only flash your headlights to let drivers know you are there, not to convey

Confusing scenarios like you mentioned are why highway code rule 110 says you should only flash your headlights to let drivers know you are there, not to convey a message, and rule 111 says never assume flashing headlights is a signal inviting you to proceed. Obviously, it's not your fault that someone else interpreted the headlight as an invitation to pull out. And indeed, it's probably one of the most widely ignored rules in the highway code. Nevertheless... it's still the other driver's fault. I've personally had it where other drivers think I've flashed them out, but I had actually just gone over a bump in the road. (I'm assuming you're British because you say junction.)