Any shares in the company that you owned prior to 31 March 1982 should be re-valued as at 31 March 1982 though I realise that will be difficult. Any increase in the value of your shares between the date of acquisition and 31 March 1982 is tax free. So, the 31 March 1982 value of the shares you owned at that date becomes your cost for CGT purposes. The shares you inherited from your parents will have a base cost equal to their value at the time you inherited them.
The reason I asked you about the nature of the business is due to the fact that where a company's business is property based, ie investment property as opposed to property development, the owner of the shares will not qualify for entrepreneur's relief when they sell their shares. If you had qualified for that relief, any CGT liability would have been limited to a rate of 10% as opposed to 18%, 28% or a combination of the two rates depending on the level of your income. You can read about entrepreneur's relief here
The only relief that you will be entitled to based on the information you have provided will be an exemption form CGT for the first £11,000 of the gain you make from selling the shares to your brother.
When you brother owns the company outright, he is free to do whatever he wants with it so long as its debts are settled and there are no outstanding legal actions, for example, or anybody who may object to the company assets being liquidated and the proceeds withdrawn.
I hope this helps but let me know if you have any further questions.