Did a bit of research (spoke to a couple of old mates from Liverpool) & the consensus is that "can't sing for tofee" comes from Liverpool where there was a famous factory making Everton Tofee - its the same reason Everton Football Club are known as the Tofees - and the practice of rewarding children who sang the hymns in Sunday School well, with a tofee. Those who didn't get one "Couldn't sing for tofee"
Its probably got some element of truth about it but who knows - its reminds me of the factory in Liverpool who were reputed to Knit Kettles from steel wool