what a waste of money
There really is no excuse for continuing with a mixture of systems. The cost to the country must be huge. To follow on Luke's example of wood: loft floorboards are still sold in 4 foot lengths. Modern houses have rafters at 600mm, so ~20cm has to be cut from every board. OK - they may be fine in older houses but the point is that the situation will never improve unless we go forward with a single system.
Honestly, who here aged under 40 can REALLY calculate accurately with imperial measurements?