Richard.W wrote a post about UK taxes, it might be worth having a read through the thread and if something is not already discussed then ask there:
There is a question on the “Dividend Tax” section regarding “What about ETFs?” that discusses this and it mentions that even distributing ETFs have “Excess Reportable Income” (ERI).
Regarding how those dividends then affect the price of the assets for the purpose of capital gains tax, it might be worth asking HMRC directly.
Nonetheless, I did a quick search online and I found this article from JustETF which indicates that ERI can be deducted from Capital Gains, but I think it would be worth confirming with HMRC anyway:
