Capital Gains Tax on Unit Trusts - for both Income Units and Accumulation Units

I have assumed income tax is payable on distributions from Unit Trusts for both Income Units and Accumulation Units? (Since it was no longer taxed at source.) I have a small number of Unit Trusts both Accumulation and Reinvested Income bearing which I have held since 1980. The original cost was about £2,000. No further amounts have been withdrawn or introduced and they are now worth about £26,000 If I sell all of them in one tax year the gain would therefore be £24,000. Am I right in assuming the tax liability would be £24,000 minus £6,000 allowance? Is there any allowance for inflation? Are the distributions, which are now included in the total value of the Unit Trusts, also liable for Capital Gains Tax, even though they have already been taxed as Income? If the distributions are separated from the capital amounts how is that done? (I don't myself have records going back to the start.)

Hi Vivien,
The capital gain annual exemption is now reduced to £3000 for 24/25.
For indexation, please refer to:
CG17200 - Indexation: inflation and CGT
The distributions are not part of the gain.
Thank you.