Common sense tells you that a single long line feeding into a group of cashiers will:
1) Make it a truly FIFO system which is fair... and us Brits love two things, queues and fairness.
2) Eliminate the hassle of trying to pick, and then change to the fastest queue... only to change back to the first after some dufus screws up the queue with a price-check item.
I'm confused as to why all stores don't adopt this approach these days. Maybe not Ikea, as jostling that huge trolley full of furniture may be a bit much... but if I can get through a windy queue with my suitcase at the airport, then I sure can do the same with my grocery shopping.
Do not underestimate the value of print statements for debugging.