Consider taking a margin loan against your portfolio and keeping it in your bank for the required time, then you just have to pay a small amount of interest and not liquidate your investments.
I also ran a little test on the "place of issue" field. On my DTV application in Jakarta, I put "HMPO" which is what it says in the authority field of my passport. On my wife's application I just put "United Kingdom", even though her passport says HMPO too. Both were approved.
Export your July statement up to today from your banking app if you're able to, including the £12k top up. Then combine 3-4 months statements + currency conversion screenshot and send it back to them.
Personally, I would top up the account to at least £12k to be on the safe side, then resubmit the bank statements along with a currency conversion from today.