There is no requirement for anyone wishing to take the ISTQB Advanced exams to have taken the BCS Intermediate Certificate.
It is an optional step which some testers may find helpful in reaching the knowledge required to pass the ISTQB Advanced exams.
ISTQB class the BCS Intermediate Certificate as complementary, meaning that is complements the ISTQB Certified Tester Scheme.
If you do decide that you would like to take the BCS Intermediate Certificate, more information, syllabus, costs and exam format can be found here