You have to configure parallel currency in FI, i.e. GBP should be your second currency. Then in non-leading ledger assign the currency GBP.
Path for Financials in parallel currency:
SPRO -> Financial Accounting -> Financial Accounting Global Settings -> Company Code -> Parallel Currencies -> Define Additional Local Currencies
As you have configured New GL, you can define two additional local currencies for leading ledger:
SPRO --> Financial Accounting (New) --> FA global settings (New) --> Ledger--> Define currencies of Leading ledger.
Further you can define and activate Non leading ledgers. Here you define two additional currencies and you can also define Fiscal year variant and posting period variant if they are different from that of Leading ledger.
Example: AUD parallel currency is EUR
