Depends on the person. This has happened to me many times so... If the person looks weird and creepy I'll just look at him bitchy or not look at him at all and walk away. If the person looks normal I'll say "Fine, thanks." and if he looks harmless I'll even add "What about you?" and that's it.
Get ready to deck them of course. There are no nice strangers where I live, people striking up conversations usually want to mug you. If they don't want to mug you they're psychos, like those people who talk at bus stops.
I've lived in the UK for the entirety of my life, I've found that I don't ignore strangers. If someone greets me first, I am normally grabbing their hand and answering back warmly, normally it scares them out of more contact.