You're taxable on the gain, being the difference between acquisition price and sale proceeds (less costs of sale and purchase). This is often complicated by the fact that income may have been reinvested which increases your original costs. If you're working with an investment manager they should be able to tell you the taxable gain.
Your first £12,000 of gains is tax free, gains after this will initially be taxable at 10% on investments until you've used up the rest of your basic rate band (so if you earn £30,000 you've got another £20,000 of gains before you fill your £50,000 basic rate band). After that the gains at taxable at 20%.
However, any investments held in an ISA wrapper are exempt from income tax and CGT.
There's no capital gains on death, although there is inheritance tax (which is at 40%, but with higher allowances). Your beneficiaries will receive the investments at current market value when you die so they will only pay CGT on future growth.
Your personal IHT position is that you have a £325,000 nil rate band, plus £150,000 residence nil rate band (rising to £175,000) if you leave your home to your kids/grandkids, so effectively £500k tax free from next year.
If you are widowed and your spouse left everything to you the above allowances could be doubled (although the double residence nil rate band would be capped at the value of your home - £300k) resulting in no IHT due (assuming your investments and home are the only major assets).