Making an offer on a house..

Noona • 🇱🇧💍👶🏻👶🏻🇬🇧

I'm a first time buyer and have NO CLUE what I am doing.. we've seen a lovely house that we want to make an offer on and the ad says "offers over £140k". Obviously I want to save as much as possible, but I have no idea what to start my offer at? Is it reasonable for me give a slightly lower offer? Like £138k? What would you offer?