If a Unit Trust/OEIC/GIA is encashed to meet expenses in a particular year, how is the gain taxed?

We calculate what the capital gain is as a percentage of the whole investment at the time of withdrawal and use this percentage to calculate the capital gain on the amount of the investment that is being withdrawn. 

We assume a full Capital gains tax allowance is available each year, increasing at CPI, and that the gains are taxed at the highest rate of 20%, or 28% if property.