A profit on the sale of shares is liable to tax for profits above the basic tax-free allowance. This is a pretty handsome chunk as far as most small investors are concerned. CGT is a problem only for people dealing in largish amounts or who have really struck it lucky with one of their stocks. In other words, if the profit is so great that CGT is due, you have done so well that a bit for the exchequer seems less painful. Windfall shares received from demutualized building societies or insurance companies are counted as having cost nothing and anything made from their sale is counted as a capital gain – unless they have been put into a tax-sheltered scheme such as a SIPP or ISA.
There is a tapered tax relief, however, so holding shares for a long time will reduce the tax liability. If the shares were bought before April 1998 the price rise can be adjusted for inflation before tax is payable. Dealing costs in buying and selling are allowable against the total gain, and there is an allowance for part-paid shares. Gifts between spouses are tax-free, so the portfolio can be adjusted to benefit from the maximum allowance.
Unquoted shares are a problem since there is no publicly available unequivocal price for dealing. For these deals you will just have to haggle with the local HMRC office.
As is only fair, losses made from selling shares in the same tax year can be set off against the profit. And if any of the companies actually go bust, the shares are reckoned to have been sold at that date for nothing and the capital loss from the purchase price can also be set off against gains.
All this may help with decisions between alternative courses of buying and selling, though once again being guided purely by tax differences is usually a mistake. A good accountant can advise on such things at a relatively low cost.