Yes it would be an unauthorised absence unless the school agrees to you taking your children out of school for the day. You may find the school is happy for your children to have the day off if you ask, in which case it will be an authorised absence.
We have really struggled this term as we were all poorly over last holiday and I work weekends and evening so when I spotted that last day of term was a movie day I was thinking that we might be better spending the day as another family day out visiting somewhere educational or fun without the queues etc. Do I assume this would still be an unauthorized absense etc even though nothing educational is happening at school???