by choosing market value, your accountant is crystallizing & making taxable the unrealized gains. that said, it keeps you up to date with your tax obligations (although it does not maximize your short term cash reserves)
but, yes, you can choose market value or cost price for any share, including valuing differently shares in the same company
what carries into the following year is the value of the closing stock as 'opening stock'. apart from that, you, in the following year, you can value the closing stock in any way you wish
for a company it is the same.
also, while not related to the topic, companies can't get the 50% CGT discount anyway for being an investor
