You will need to refer to RDR1 here
which deals with residence, domicile and remittances.
The fact that your mother is a UK citizen doesn't necessarily make her UK domiciled. If she isn't UK domiciled, then she can choose to leave the disposal proceeds of the land abroad and pay no UK tax. However, as she is a long term UK resident, that will leave her liable to the remittance basis charge for any tax year she chooses to be taxed on the remittance basis which you can read about here
If she chose to bring the money into the UK or is UK domiciled in which case it doesn't matter if the money is brought to the UK or not, then any gain will be subject to CGT in the UK. For this purpose, the 31 March 1982 value of the property will be the cost for CGT purposes. There are two rates of CGT, 18% and 28%. The rate or combination of rates paid will be dependent on the level of the individual's income in the tax year the gain is made.
The disposal proceeds will become part of your mother's estate for Inheritance Tax purposes. She has been in the UK for 40 years so she is deemed to be UK domiciled for IHT purposes. Should she then give the cash away, she will be making gifts which will cease to be part of her estate should she live for seven years after making them. A form of tapering can reduce IHT liabilities on gifts made in the seven years before death as you can read here
. A term assurance policy can be used to cover an IHT liability but the older the life assured the more expensive it will be. The gift recipients will not be liable to UK tax unless the donor (your mother) died within seven years and the estate could not pay all the IHT due.
I hope this helps but let me know if you have any further questions.