Thank you Nik and Adam for the clarifications on this subject which hopefully will be helpful to others as well.
I have funds in GIA which, between my OH and myself, we may not spend all in our lifetime so would look to leave this to our son in the future. It's a fine balancing act between whether to annually make withdrawals to fund the £20,000 ISA allowance or just withdraw up to the piddly £3000 annual gains allowance (e.g. £15,000 withdrawal if gains are 25% - £12,000 capital + £3000 gain).
This saves the hassle of doing a tax return since my pension is PAYE so HMRC adjusts my allowance for interest income annually.
Who knows, the govt may change this in future but it would seem harsh to pay gains tax followed by IHT.