No, you don't have to move to the US to get representation or sell a script. When I was at CAA we repped many a artists in the UK and Australia. The best way to go about it, is to find an attorney in Los Angeles that is willing to represent you. It is hard without having something sold or in the process of being sold. If you have a great piece (and I emphasize GREAT), then find some attachments to it first. Get interest from an actor, director, or producer that has a market pull and then when they are interested in taking on your script in a project contact an attorney to represent you or a medium agency. I hope this helps and above all be patient and persistent in your craft. If you really want to be a writer and constantly evolve and learn, then you will find a way in or eventually be read by the right person.