I've just tried, using Chrome browser on a macbook air.

Using the following link https://www.britishairways.com/trave..._gb?eId=106019 allowed me, to choose more than 1 voucher at the same time. (However as there's only 2 in my household, t wouldn't let me proceed any further searching for 4 passengers, as only 2 registered in my household )

Possibly worth a try to see if it works for you?