I have decided to reduce the width of the tall larder unit to the right of my row of appliances from 600 to 500mm. This gives me much more space between the machines and will allow the use of decor panels as intermediate worktop supports.
I have also reached the conclusion that a perfect solution for the power feed to each appliance simply does not exist! I had been tempted to go for replacement longer mains leads that let me connect the plugs to switched sockets conveniently located in the larder unit, but I have now realised that I would probably have to pull all three machines out from under the worktop if I wanted to fully withdraw any given cable, as otherwise the passage of the 13A plug would be blocked in several places.
Every solution has issues, but cutting off the plugs and hard wiring to a flex outlet behind each machine (fed from DP switches in the larder unit) is once again in the frame, as perhaps the least troublesome.