UCU’s new advice to members is as follows:

Do not respond to requests about your participation until the action has begun.
As soon as the action begins (e.g 28 April), if asked by a person in authority members should answer truthfully. If you have a discretion as to when you start marking, a reply such as this below would be recommended.

“I intend to support the UCU marking boycott but will not be doing so until x date when I have scheduled work to do which is included in the boycott. I will not be engaged in marking every day after that date. Please note that I would have expected all my marking work to have been completed by x date.”

If you do not have discretion as to when you must start marking, then reply giving the date when you would have started. If you know when your marking work would have been completed, you should also give that date.