Due to BREXIT need to provide UK VAT in sterling amount for EUR invoice in EUR functional books

We are shipping goods into UK but have agreement with customer to bill them in EUR and customer pays us in EUR. And our Functional currency is EUR. But as per HMRC, we must provide the total amount of VAT in GBP. We can customize Invoice print program to display both EUR & GBP value on the basis of Corporate rate. But like to know how we can have dual values of tax amount one for receivables in EUR to collect & another for VAT tax liability in GBP for payment. Issue is recording & accounting of tax liability. We are