Understand the tax benefits of having the interest paid into the ISA but everyone may have a different investment strategy on the GIA vs. ISA. Mainly for me the ISA has some longer term investments. The GIA has riskier positions in the short term and would rather the interest payments reinvested here as these stocks are not available within an ISA.
Not saying this is everyones investment strategy but giving the flexibility of where interest is paid based on their circumstances. For example, never heard of a bank charging overdraft fees to the credit card you also hold with them, as they are separate products…
If you have £2k in your ISA and £2k in your GIA then you will get 50% interest paid into each account, as :freetrade: states:
We will give preference to your ISA followed by your GIA.
Let’s say you are waiting to invest £5,000 across your two Freetrade accounts: you have £3,000 cash in your ISA and £2,000 in your GIA. We will apply the interest to the full £3,000 in the ISA first, and then to the £1,000 remaining in your £4,000 cash cap from the GIA.
The interest payment will then be split accordingly into the two accounts.
The only way is to reduce your GIA account holdings so interest is paid to your GIA and ISA? A toggle to set your preference with ISA as default would be ideal 
Assuming I’m in the minority requesting this feature as I searched other threads before so never likely to happen.