When dealing with invoicing in other currencies you first need to apply a different currency on the Assignment please see the following article How do I apply a different currency on assignments timesheets
Please also see the following article on exchange rates How do I set up exchange rates on Etz
We suggest you update these exchange rates on the same day each week i.e. (Thursday).
We also have a number of System Invoice Templates which you can use to produce your Invoices.
You can assign a distinct template on a client record.
When creating your sales and purchase batches we would recommend batching together all the Euro transactions and then creating a separate batch for your GBP transactions.