Dear Seena,
It is very much legal to install CCTV cameras inside the shop floor. You require to notify the employees about this and need to display a poster saying "Property under electronic surveyllance". You need not worry about the unions, Those issues will always be there. You need to explain the unions the purpose of the CCTV installation.
Many company's are installing CCTVs at shop floors not only for security reasons but also for safety and productivity purposes. CCTVs can be used for doing JSA ( Job Safety Analysis), to correct any safety violations, to maintain hazard free working environment, for time and motion study etc.
Your intentions should be clear so as to convince the unions. If you are sending signals that the CCTVs will be installed to spy on employees/workers then your task will become difficult.
Regards
Anjan