You must report the gift of the computers on the HM Revenue & Customs form P11D and the business will incur Class 1A National Insurance (unless the employees are non-directors and earning less than £8,500 p.a. in which case use form P9D).
I assume the use of these computers has not previously triggered a tax charge on the employees. On this basis, the value to declare on the form is the second-hand value of the computers when you give them away. Please note, if you are VAT registered, you must include VAT on top of the market value.
If the employee is a director or they are earning in excess of £8,500, they will have income tax to pay on the gift. If they are not, they will have no income tax or National Insurance to pay.
The implications of gifting assets to your employees can be tricky as there are different valuations to use depending on who the recipient is, if the asset has previously triggered tax charges on the recipient and whether the asset has appreciated in value. So before you proceed with making any decisions about giving assets to staff, I would encourage you to speak with your local TaxAssist Accountant so you are fully aware of the consequences.