AFAIK there's no legal right to organise/attend union meetings in work hours. The only exception might be a collective redundancy where staff are entitled to consultation time during work hours.
If it's a unionised workplace and they have a prior agreement with the employer, then perhaps it would be allowed, but I'd be very surprised if an employer had a blanket agreement to allow meetings without notice.
All out union meetings take part at lunchtime or after work unless in the past where is was to do with collective redundancies and those staff had more access and a lot more meeting but for regular meeting all outside work or lunchtime