The concept of wash sale exists in the US tax law because the US taxes capital gains differently depending on the holding period. Thus selling at a loss to offset gain that is taxed at higher rates may be beneficial even if you intend to hold the position longer by repurchasing it back.
In the UK, capital gains are taxed at different rates based on the overall income (similarly to the US), but there's no meaning to the holding period (unlike the US).
There's, however, a rule that forbids recognition of loss if the same class of asset is repurchased within 30 days after the disposition. This is similar to the US wash sale rules, but not quite. For example, the US rules define the recapture period as 30 days before and after the disposition, while the UK rule only looks at the 30 days after. This means that your tax loss harvesting strategy would include repurchase first, disposition later (that doesn't allow to harvest all of the loss necessarily, but some of it). You cannot repurchase and dispose on the same day due to a different rule. Additional tax loss harvesting strategies may include CFDs, described here.
Here's more details about the UK taxing of capital gains, losses, and the capital gains taxes rates.
In some other countries I'm familiar with you can even do "conceptual sale" where you don't actually sell, but report taxes as if you sold and repurchased - specifically for tax loss harvesting purposes. You'd just reset your basis to lower amount and report losses to reduce your taxable income. I'm not sure if it is possible in the UK.
