What are the financial offer or opportunities for international PhD students in US and UK?
In the US, financial aid from the US government under Title IV of the Higher Education Act (student loans, Pell grants, and work study) is available only to US citizens and permanent residents (and a small number of other categories of people who have some permanent status in the US.)  Generally, foreign graduate students aren't eligible for this kind of aid.  

Typically, individual academic departments make decisions on who to admit and who to offer teaching/research assistantships.  If a department decides to offer you admission with an assistantship, you'll typically receive an offer letter from the institution describing the aid in detail.