isupplier invoices default tax

Hi

Isupplier user logs in to the isupplier portal and creates invoice

The invoice is in EUR and our functional currency is GBP

While testing this scenario the tax always defaults to GBP 20

We cannot change this to GBP zero

Question

1. How to set the default tax to zero

2. If not how can the tax be changed

3. Where are the default tax rates set. Is it derived from PO

Thanks

Ninad