Preview an Unpublished Checkout in Multi-Currency?

We are looking to test an unpublished checkout (one-page) and wanted to test in the shop’s markets configurations.

Example: shop is based in USA, want to preview the one-page checkout in GBP for the UK.

I tried updating the URL params but nothing is sticking:
&locale=en-GB&country=GB&currency=GBP etc..

Anyone have a workaround here? (I also tried VPN)