Under normal civil law, a trade union would be acting unlawfully by encouraging and inducing people to breach their contracts of employment. The employees in question would also normally be acting in breach of contract and therefore also potentially subject to civil action.
In order to get around this there are special laws in relation to trade unions which allows them to induce people into breaching their contracts, provided certain conditions are met. If these conditions are not met, then the industrial action could be unlawful.   The conditions that need to be met include the need for there to be a ballot and for employees to be in favour of the action. The recent planned British Airways strike was found to be unlawful as it was held that the trade union had balloted a number of people who should not have been balloted. This shows that these rules are inflexible in their application; although the additional balloting would have made no difference to the outcome of the ballot, there was still a massive majority in favour of strike action, it was still ruled to be illegal.
There are a number of other requirements that need to be met which include the requirement that there is a trade dispute, that the industrial action is not in support of another employer, and that notice of the intended action is given to the employer.   As an employer, when you first become aware of the prospect of industrial action, you need to ensure that you obtain legal advice from your employment solicitors. It may be possible, where there are illegalities in the industrial action planned, to get an injunction from a court to stop the action taking place. It may also be possible to use employment solicitors when negotiating to prevent the action and when attempting to make changes to the contracts of employees.  
It is rarely beneficial to have the strike action take place and it will usually be favourable to negotiate changes to contracts. In many cases, having a good team of negotiators and some flexibility towards the planned changes will make the difference when attempting to avoid industrial action.   As an employee, your employment rights are protected by employment law in regard to lawful industrial action. Therefore, it important to obtain legal advice from an employment solicitor if you are unsure of the legality of any industrial action you are being asked to take part in.