So, great question, generally in the UK:
Teacher is someone who teaches people at school up to about 18.
Lecturer is someone who teaches people at University so roughly 18-25
Instructor can be someone who is teaching a skill so welding/car driving/ swimming etc. But they can also be say second degree students who are coaching first degree student in technical subjects in a laboratory.
But there will always be individuals who break the rules for their own reasons.
Professors have a "chair" which means they get specific funding and can focus on specific areas of research. They may or may not be heads of department, but a head of department will be a Professor.
You also need to understand the difference between a college and a University in the UK and the USA, since I don't understand the US system I cannot comment.
In the UK the term instructor is not a general term for a part time employee on the teaching staff of a University but it does designate a junior position and may be part time.