You can value your 'closing stock' in one of three ways: cost...

You can value your 'closing stock' in one of three ways: cost price, market value or replacement cost. You can choose whichever valuation method is most advantageous to you for each parcel of shares. However, you must then use those values as the opening values for your tax return the following year.

It's all explained in more detail in Tony Compton's book "Shares, Derivatives and Taxation". Recommended.