Valuable possessions such as antiques and collectibles are called 'chattels' in tax speak.
Any profit you make from selling certain types of possessions is tax-free; items with a predicted life of 50 years or fewer, known as 'wasting assets', are CGT-free, provided they were not eligible for business capital allowances.
Antique clocks and vintage cars are treated as 'wasting assets', as are pleasure boats and caravans.
If your gain is not tax-free, capital gains tax is charged in a special way. Your taxable gain is the lower of the actual gain, or five thirds (166%) of the excess of the final value over £6,000.
So, if you sell a pair of antique candlesticks that you originally bought for £5,000 for £7,000, the actual gain is £7,000 - £5,000 = £2,000. The gain under the special rules is 5/3 x (£7,000 - £6,000) = £1,666. Since this is lower, your taxable gain is £1,666.