UK has different taxes (VAT, etc) and I don't know if it can represent the invoices accurately that were made in the US (which QuickBooks Mac assumes). So that's the first issue - can you even create a duplicate of a US invoice from the Mac manually by typing it into the UK QuickBooks Online?
Assuming that can be surmounted, the approach I'd try is QuickBooks Mac export to QuickBooks Online US. And then see if there is a way to migrate QuickBooks Online US to QuickBooks Online UK. Since the first step (Mac->QuickBooks Online) is doable you have to solve the second (QB online US -> QB Online UK). So research that second step and see if there is a published method.