What options does an international PhD student in UK have to fund his PhD?
Do TA. The easiest way is to do TA for the courses that your advisor or professors in your group teach. I was paid £12.XX per hour at my school in London. As long as it is less than personal allowance (£7000 per year), you don't have to pay tax or anything, and it can be added on top of your scholarship, so it's good.
Find an internship. My scholarship prohibited me from taking internship so I was kind of happy when it expired.
If you major is CS, there is some programs like Google Summer of Code (open in February), which you can earn > $5000 for 3 months with no tax.