Alex, I have only one thing to add to May's excellent answer. The fact that a woman is a looker is not the sort of information that one would normally pass on as a matter of business (unless the people having this conversation are in a business related to beautiful women). In a conversation between two ordinary people the response, "I'll be 'certain' to pass that on," probably is intended to be humorous. Most likely the information that "she is a looker" will not 'officially' be passed on to collegues.

The term 'she's a looker that one' is an expression used in english, particularly the UK. It's slang or colloquial phrase and is used to refer to a somebody's beauty. It practically means 'she's very pretty'. The word looker is used in this context as the amount of attention [this] female gets. You can substitute the word 'she' with 'he' when referring to an attractive male. Your second sentence ' I'll be certain to pass that on' refers to a context in which one has just been given some information where they've either been explicity or implicity asked to pass some information on to someone else. The sentence therefore is a confirmation that you will tell whatever you've just been told to the other person. To translate into simpler english it basically means 'I'll tell them what you've just told me'. So for example, you're working in a business and your boss tells you that there's a new deadline for your team, you would say 'I'll be certain to pass that one' meaning you'll tell the other members of your team that there's a new deadline. I hope that this has made it a bit clearer for you. Please let me know if you need any further explanation ^_^