HI Tom,
The isa in essence is a wrapper for your investments that shields them from capital gains and dividend taxes.
You do have annual allowances that you can earn before things are taxed but if you’re investing over the long term, you’re best off putting as many things into the isa as possible rather than in the GIA which could attract tax.
Don’t forget tax rules can change so annual allowances as they are now, can go up or down.