Unfortunately capital gains tax is not as simple if youāve made hundreds of transactions outside of your general account.  I do to and I have a google spreadsheet tracking everything up!
Note that this is general advice and Iām just doing some illustrations to show the complexities of apply the rules. You should get independent tax advice if you need to!
1. ISA is nontaxable
As youāve said, your investments is in a general account (i.e. NonISA), because any profits / dividends is tax free within the ISA (Although if you do make a loss then you wonāt get tax relief either).
2. No Sale = No Gain
Your account balance reflects the total value of your portfolio. So unfortunately the gain is not as simple as your closing balance less your opening balance, as youāve said in one of the threads.
3. Report Threshold
As @rivorson said, you will have to report capital gains for the tax year if one of these are true:
a. Realised gains (profit) exceed the annual allowance Ā£12,300 for 20/21.
b. The total sale proceeds (i.e. all the āSell transactionsā Ā£ amounts added up) adds up to more than 4x the annual allowance.
c. You want to report any capital losses to be carried forward to future tax years.
4. How gains are calculated
Whenever you sell any shares:
i) you sell the shares that you acquired at the same day first.
ii) then you sell the shares youāve acquired within 30 days on a first in first out basis,
iii) anything else is left in the general pool of shares youāve acquired.
As an illustrative example, say someone made the following transactions within the tax year:
01May, Purchased 600 shares at Ā£10 each. (Total Ā£6,000)
02May, Purchased 400 shares at Ā£11 each (Total Ā£4,400)
15May AM, Purchased 500 shares at Ā£9 each (Total Ā£4,500)
15May PM, Sold 50 shares at Ā£10 each (Total Ā£500)  holding 1,450 shares
03Jun, Sold 500 shares at Ā£12 each (Total sale proceeds Ā£6,000)  holding 1,000 shares
14Jun, Sold 250 shares at Ā£9 each (Total Sale proceeds Ā£2,250)  holding 750 shares
15Jun, Bought 1,250 shares at Ā£11 each (Total Ā£13,750)  holding 2,000 shares
July, Bought 2,000 shares at Ā£15 each (Total Ā£30,000)  Holding 4,000 shares
October  Sold all 4,000 shares at Ā£14 each (Total sale proceeds Ā£56,000)  Nil Shares left
Gain made on:
15May, This lot is counted against the purchases weāve made in the morning, so the gain is (Ā£10  Ā£9) * 50 shares = Ā£50. Note that if the sale was in the morning and purchase was at the evening, then the gain would be zero. (look back 30 days, 01May & 02May, First in First out, therefore look at 01May firstā¦ (Ā£10Ā£10) * 50 shares = 0)
03Jun
No shares purchased on that day, so you go back 30 days and find the latest acquisition, which was on 15May. There are 450 of the 15May AM shares left, so you count against them first. No other shares purchased 30 days prior to the sale, so look at general pool for the remaining 50 sharesā¦
First the 450 shares at a cost of Ā£9 each = Ā£4,050.
Then look for the remaining 50 shares from the general pool, (Ā£6,000 + Ā£4,400) / 1000 shares = Ā£10.40 per shareā¦
Ā£10.40 * 50 shares = Ā£520.
Total cost of shares = Ā£4,570.
Sale Proceeds = Ā£6,000.
Therefore Gain = Ā£1,430.
14Jun
Simply take the 250 shares from the general pool
Cost = Ā£10.40 * 250 shares = Ā£2,600
Sale Proceeds = Ā£2,250
Loss of = (350)
October
Now we are just selling all of our shares, so the cost is:
750 shares in General pool (as calculated above) = 750 * Ā£10.40 = Ā£7,800
1,250 shares bought on 15Jun = Ā£13,750
2,000 shares bought in July = Ā£30,000
Total Cost = Ā£51,550
Sale Proceeds = Ā£56,000
Gain of: Ā£4,450
Summary:
So therefore, just by looking at shares of company A, assuming the person have not made other asset disposals subject to capital gains tax, then

In the period, weāve disposed shares with value of Ā£64,250, which is over Ā£49,200, so we will have to fill in a tax return.

The total gain is Ā£5,530, which is below the Ā£12,300 annual allowance limit, and therefore no CGT is payable.

Obviously things get more complicated if thereās stock splits etc!

Therefore depending on your circumstances, you might have to get some professional advice if you donāt know what youāre doing!

You can find out about the rules at: https://www.gov.uk/capitalgainstax
Hope this helps