Provided your union is independent and recognised by your employer, you are entitled to reasonable time off with pay for union duties. The Acas Code of Practice on Time Off for Trade Union Duties and Activities (PDF, 199KB) contains some examples of duties and activities (e.g. negotiating meetings, some forms of training, etc.).
Union members are also entitled to reasonable time off for union activities. Your employer does not have to pay for this time off. Such time off may include attending union information meetings.
Rights to trade union facility time are under sustained government attack, especially in the public sector. TUC Education has developed a set of online resources for reps (called eNotes) to help you learn more about and defend these rights.