Not a clue about how hard it is for international students to find work, although even British students can struggle. You'll do best if your English is fluent and you have some experience in a student-type job - bar work, waiting tables etc.
Minimum wage for people aged 21+ is £6.08, and for those aged 18-20 it's £4.98. However, even as an inexperienced 18 year old straight out of school, I always refused to accept anything less than the 21+ rate, and have always been in work when I've wanted to be.
I think the university recommends that you work no more than 16 hours a week during term time, but in my opinion that is too much, and you may find that you have restrictions on your visa anyway.
Essentially, you'll find that part time work is never going to be anything other than a supplement to your other sources of income - student loans, money from parents etc.