Thats interesting that you got a bill for BT in arrears, they have never offered that option in England. Its always been line rental etc in advance, usage in arrears.
Not optional and always been the same even going back to GPO
Your also paying for some fixed element in arrears with utilities, standing charges.
Its still not the point, they can't provide the energy for free they need to pay for it.
If everyone paid in arrears, then the chances are the prices would be higher since the companies would either need to borrow or expect shareholders to fund the energy purchases whilst waiting for customers pay yeah (yeah right!)
I remember well when it was normal to pay in arrears for your bill, people would complain all the time about getting random amounts with just 14 days to pay, "not even getting paid before this is due" that sort of thing.
Simple fact is for most people its easier to budget costs into equal buckets since they get paid in equal buckets.
Its more about the accuracy than the timing.