I have two iMacs, which are well used and five years old. Am I allowed to bring them into the UK for free, given their age, or do I have to pay VAT or something else at the border at Heathrow?
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.