If you are in doubt you should ask your head of department whether attendance is voluntary or mandatory. If they say you must attend, ask them to put in writing direct to you and take it up with your rep. A written instruction to you does not count as a general instruction to all, so other members should seek individual confirmation that they are also required to attend.
You could write using the following formulation:
“As you know, UCU members are currently working to contract and this action includes non-attendance at voluntary meetings. Consequently, I am writing to ask you to confirm in writing whether you require my attendance at [name of meeting].”