Before you can understand how to express your disapproval, you need to understand a little bit about how the UCU (the Union organizing the strike) and universities work.
I have not personally been affected too much yet, but have had one module's continuous assessment removed with no suggestion of replacement.
It is not the responsibility of those on strike to come up with the alternative. It is the responsibility of the University who is collecting your fees to deliver what it has promised.
The union in question have poorly planned this boycott and there are currently no plans to remove it. We have had no communication about who is striking and what their alternative plans are if this continues and I am very concerned about this.
The marking boycott has been in the works for months now. The universities and union initially had extended discussions, but were not able to reach a resolution. The union then suggested if an agreement could not be reached, that they would recommend a marking boycott. After additional talks failed to reach a resolution, the union brought asked its members to vote on a marking boycott. After the marking boycott was approved by the union members, a final round of discussions were held. After those discussions failed to reach a resolution the marking boycott was initiated. The union is not allowed to contact students. It is the university's responsibility to contact students and tell them what is happening and what the alternative plans are. Issues about lack of communication and alternative plans should be addressed to the University.
I want to express my disapproval with their methods as I disagree entirely with the boycott as I believe using students as pawns is never acceptable. For example they have been told "Examination of dissertations and theses at postgraduate level, as well as vivas, are included in the action." which is much too far. I don't want to damage the relationships I have with my lecturers as I plan on staying in academia, but seeing these actions is making me question my desire to stay in academia.
This is something that should be directed at the union. The union chose what they are boycotting. The union could have used a research boycott, a recruiting boycott, or a teaching boycott, but the union chose to focus on current students.
Your student union may be able to help you voice your concerns to both the university and the union. It is possible that your student union is backing either the union or university and will not pass on criticism to either. If you want to contact someone directly, the UCU is suggesting students contact the vice chancellor's office at their university. To contact the UCU you could use anyone on the UCU contacts page.