So I thought I'd try doing the not choose the vouchers till I was in the store thing - only it turns out the last possible day for choosing any given week's vouchers is the Tuesday - but the next set of vouchers don't become claimable till the Thursday.
So if you a week when Stuff and Things mean you don't get into town to do a shop until Wednesday, if you haven't chosen your vouchers in advance it's too late....

[It's also slightly awkward pulling out a tablet at the tills to do the voucher transactions - they obviously expect people to use a smartphone but the screen on that is too small for my eyes/reading-issues to cope with that level of detail]