Clean Capital Separate Account

I have a very basic question about the mechanics of creating a ring-fenced account for clean capital.
I will be selling a mixed fund at Vanguard to separate the clean capital from the gains, and need to be sure I do it correctly.  I plan to exchange all shares into my Vanguard Prime Money Market account, and leave there for the time being.  When ready to transfer to the UK, I will do the accounting to separate the "clean capital" from the gains and open a non-interest bearing checking account for the specific purpose of keeping the clean capital separate. Once it has cleared that account and I am ready to send to the UK I will do so. Does that sound right?

Is the proposal to use the two year “cleansing” window for mixed funds, for 2017/18 and 2018/19? This will not be possible under 6 April 2017.
The explanatory notes to the Finance Bill 2017 can be see here
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/574679/Explanatory_Notes_-_draft_provisions.pdf
The cleansing rules, including the need to make a nomination, are set out at page 166 et seq. It should be remembered that changes to these proposed rules could be made before they are finalised.

Is the proposal to use the two year “cleansing” window for mixed funds, for 2017/18 and 2018/19? This will not be possible under 6 April 2017.

Yes, that is correct.  I am thinking of selling now, and then doing the cleansing accounting after April 6.
Thank you for the link to the document. Could you explain what a "nomination" is?  I did not see any reference to it in the document.

The nomination is mentioned in the fourth bullet point of paragraph 99 on page 166 of the notes.

Got it. Thank you.