Computers, Laptops and even mobile phones can view live television as it is broadcast across the internet. If you or other people within your organisation watch Live TV then you need to buy a licence.
The key here is LIVE TV, so if you watch the lunchtime news live from your PC or Laptop then you should have a licence.
Unless you already have a license I would suggest that you add into your acceptable use policy a statement that says that it is not permitted to watch Live TV from a Computer within your organisation. So when the letter arrives, which is apparantly being sent to all addresses without a TV Licence, you can let them know that you have taken steps to prohibit people from watching TV.
However, if a person is watching on a mobile device, operating by battery, then it is the individual who should have a licence. If they plug it in then it becomes the organisation who is responsible.