Possibly Too Much Information, but what you can do (usually) is limit the spread of the sell from the GIA to the ISA as it’s a zero risk trade for the market makers. You’re performing what is weirdly known as a “Bed and ISA” - simply a GIA to ISA transfer but facilitated via a Sell and Buy in order to generate the correct tax liability (if any) in your GIA and to account for your ISA allowance on the inward leg.
If you call up (argh! Voice Trade - they don’t happen anymore) the market maker will often reduce the spread to virtually zero for you as there’s no risk to their position.
Pointless in the case of a highly liquid stock and you’re transferring a small position. Essential if you’re moving an illiquid stock with a wide spread.