I have not known about highly trusted universities or colleges offering art courses to international students in around £3000 a year in London (not even in UK). Good universities and colleges in UK have at least £6000 as a tuition fees per year for international students. I can provide you list of some cheapest universities and colleges in UK for your help. Review them.

List of Cheapest Universities in UK offering Art Courses

Heythrop College, University of London
http://www.heythrop.ac.uk/

UHI Millennium Institute
http://www.uhi.ac.uk/

Glyndwr University
http://www.newi.ac.uk/

Liverpool Hope University
http://www.hope.ac.uk/

York St John University
http://www.yorksj.ac.uk/

University College Birmingham
http://www.ucb.ac.uk/

City College Norwich
http://www.ccn.ac.uk/

St Mary's University College, Twickenham
http://www.smuc.ac.uk/

Writtle College, Chelmsford
http://www.writtle.ac.uk/

University of Wales, Newport
http://www.newport.ac.uk/

Leeds Trinity University College
http://www.leedstrinity.ac.uk/

Thames Valley University
http://www.tvu.ac.uk/