If you have made international transactions from your GBP account, this will feed through into your VAT calculation based on the exchange rate you were given for the transaction. If you’ve supplied/purchased goods to/from the EU, or services from outside the UK, then you may need to report some additional information to HMRC. The VAT calculation tool supports this type of cross border VAT reporting.
The VAT calculation tool doesn’t support non-GBP accounts, so the transactions in your Starling euro business account aren’t included in transactions shown in the business toolkit. However, you can add these in as ‘external transactions’ in the bookkeeping section, once you’ve applied the relevant HMRC foreign exchange rate to them.