I think I may have been wrong (I did say that I might be). The Gov website says
" If your total taxable gain is still above the tax-free allowance, you can deduct unused losses from previous tax years. If they reduce your gain to the tax-free allowance, you can carry forward the remaining losses to a future tax year."
Capital Gains Tax: what you pay it on, rates and allowances: If you make a loss - GOV.UK