How & Why can a Company be listed in USA stockmarket & UK etc?

Can a UK based company that is listed in the UK stock exchange be listed on any of the USA exchanges and if so how and why-what benefit is that to the USA economy and to the UK company?

Does the company have to be in the USA to be listed there?

Thanks x

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).