how to be a freelancer if you not from the uk and inside the country already

Hi

My name is Simon
I'm dual resident of South Africa and Israel i would like to work in the U.K as a freelancer in my field (special effects,robotic,designing products and ideas for the commercial market ).
I do have a place of address in the U.K

What would be the steps of optaining one inside the U.K.if im there already ?
How long does it take to get one?
How much money would it cost to get one?

Thank you
simon