I've finished the Performance certificate from Trinity College London, and now I would like to continue with the ATCL Diploma from ABRSM. Does it include any theory parts that I need to pass before I take the exam?
The diploma syllabus gives a full list of requirements and the equivalent qualifications from other boards. The DipABRSM requires grade 8 practical, which requires grade 5 theory. Grades from other boards are accepted but it also notes that grade 5 theory is required in addition, though from any board. All the higher diplomas require lower ones, so eventually require theory.
You can also substitute qualifications from music colleges;these generally have some theory component as well.
The syllabus is here and the information is under 'prerequisites and substitutes' (about page 20)
ABRSM also has its own forums where you can get additional information.