If I were to go to school in USA, would I still be able to get a job in the UK?

It's a degree, not a diploma, on the university level. Diploma is what you get when you complete secondary school. (We call it "high school".)

A lot of British students have chosen to study at US universities in recent years. This will not hinder them in terms of getting jobs in the UK if they study useful subjects. However, "media" is a very vague field, and it wouldn't be easy even for an American to get a job here after having read (we say "majored in") something called "media". I doubt you'd do any better in terms of employment in the UK.

Some US universities do offer financial aid to foreign students. They tend to be our more selective universities, however, which have a lot of aid money available but to which admissions are very competitive.

If you are seriously interested in studying at a US university, there are consultants in the UK who assist with applications, arranging to take the necessary entrance exams, and so forth.

Have you considered how much it will cost to study in America? Unless you happen to be independently wealthy, you may find this isn't an option.

Media isn't seen as a 'good' degree anywhere, and won't necessarily be any use in getting a job in the UK, wherever you study. However if you do study in the UK you stand a chance of networking and building useful connections, and will also be in the country to apply for work experience and internships