Upvote:3
Estimate their value (look at what they sell for on eBay, for example), and declare that at the border when you import them. They'll be tax-free if you're within your exemption amount (which will depend on whether you're a UK resident or not, and how long you've been out of the country), or at the discretion of the Border Force officer (if [s]he feels generous, you may be allowed them tax-free even if you're over your exemption).
It wouldn't hurt to print the eBay listings you find as evidence of the value, if you're asked to justify it.
Five-year-old computers still have some value, particularly if they're Macs, so I wouldn't be surprised if you have to pay a little.