Charts and base currency

Hello, our CRM was originally for our US office. The UK have subsequently come onto it so we now have a duel setup for US/UK. I have a number of dashboard charts (for the UK) but am struggling with orders and the currentcy in charts.

I have a chart that shows orders in a QTR by owner and the numbers are in USD despite our order forms all having the values entered GBP. From a bit of research, I have found a few posts that suggest that it will default to the base currency (which is USD). 

However... I also have a chart that shows opportunities by QTR by owner and this is showing in GBP

So my question is why does opportunities by QTR / user show in GDP and orders by QTR / user show in the USD base?

Is there any way to get the orders chart to show £ rather than $ the same as opportunities does?

Thanks