Most international students' passports are stamped "Leave to enter the UK on condition that the holder does not enter or change employment, paid or unpaid, without the consent of the Secretary of State for Employment".
It is a serious offence to work in excess of 20 hours during termtime (vacation work is nlimited) and can lead to being asked to leave the country.
Working once studies have finished depends on whether you meet the requirements for any of the schemes that the Government operate eg.
Fresh Talent: Working in Scotland scheme (for those who have studied and lived in Scotland)
Science and Engineering Graduates Scheme (the Government changes the list of courses that qualify students for this scheme regularly; the list is very limited and changes regularly at short notice)
Work Permit scheme (for jobs where noone suitable can be recruited, and occupations where there is a shortage of qualified workers)
Training and Work Experience Scheme (for work-based training for a professional or specialist qualification, or work experience)
Highly Skilled Migrant Programme (for those with exceptional skills and experience)
the Innovators scheme (for entrepreneurs)
Contact (
http://www.ukcosa.org.uk/) for more details.