Is UK's IGCSE English first language grade of C equivalent to US's grade of B?

The University of Washington requires international students to prove their English proficiency through IELTS, TOFEL or SAT/ACT score as an alternative method.
However, the alternative method can only be used if the student has 580+ on SAT English Reading & Writing part, and grade B in high school first language English writing & literature courses.

“A minimum grade of B in all high school level English writing or literature courses where English is the primary language of instruction”

I am studying in the UK but unsure of how the conversion system works between UK & US. I have heard that grade C in IGCSE subjects is the same as grade B in US high school courses, does anyone know if that is true? And whether or not I can use that alternative method of submitting SAT only instead of taking the IELTS?

Moreover, I have contacted the international admission officer, but I did not receive an specific answer from her.

http://www.fulbright.org.uk/going-to-the-usa/pre-departure/academics

Thank you for your help , but it is not related to IGCSE grades conversion to US grades.

"International students whose primary and secondary education took place in Australia, Canada (English-speaking provinces), United Kingdom, Ireland, New Zealand, or the U.S. are exempt from this requirement. "

Is English not your first language? You have a C in IGCSE english?

Yeah, I am an international student

          What is your home country and first language? I would doubt a C as a 16 yr old exam candidate will be counted for anything. You are studying in the UK, doing A levels in English? 

No, not doing English in A-levels. I am from HK , first language is Cantonese - Chinese basically

Unfortunately, I have only studied in the UK for few years, so I won’t be exempt from that requirement.

There is a reason they have this requirement. You will need to take college level English courses and Social Studies course here in the US to fulfill your GE requirements. Even if you major in engineering or sciences.

Take the TOFEL and do well.

Yes, I have emailed the admission and my IELTS score has already met the requirement. I guess I shouldn’t stress about it too much