You should be able to buy a screw-on cap from a plumbers merchant (or maybe a DIY store) to seal off the hot valve.

In theory, just leaving the hot tap turned off and unused should be fine (and that’s what most people do) but many common taps are prone to getting accidentally knocked and allowing water to leak out. I’ve heard of cases where the lever’s been knocked inside the cupboard or even by the washing machine moving about on spin if fitted behind it causing floods. A slow dripping tap can cause a lot of damage.

The whole concept of hot and cold fill washing machines is virtually redundant and out dated. Modern washing machines use so little water that most would not take any hot water in and still have to heat it up. This is because by the time the hot water runs into the machine it’s finished filling. There is also an argument that having a hot valve is wasting hot water for most people because in most cases all that happens is the washing machine draws in a few litres of water but it’s cold because the hot water has cooled in the pipes. It stops filling just at the time that proper hot water has been drawn into the pipework but not reached the washer (or only just reached it). This hot water doesn’t get used and is usually wasted as it rapidly cools in the pipework.

Also, when supplied from a hot water cylinder, all the water drawn into the pipes is replaced by cold from the header tank in the loft, which cools down the rest of the hot water and has to be heated back up.

In this common scenario all that happens when a modern washing machine draws a couple of litres of water of hot and cold water is that none gets into the machine but hot water is wasted and replaced by cold which needs heating up again. Even with combination boilers there is still waste hot water because of cooling in pipes and it takes a lot of energy to heat water instantly as it passes through the boiler. There may well be scenarios where all this doesn’t happen but it does for the majority of people.

It’s all explained in this article but it’s lengthy - Cold fill washing machines

At the end of the day, manufacturers are falling over themselves to produce washing machines that use less energy to get the coveted A energy efficiency rating. If it used less energy using a hot valve, you would think they’d put one in. As most people only wash at 40 degrees and less, and I believe we are heading for cold water washing anyway I can’t see the hot valve coming back

I do believe it is possible to use one - particularly for the boil or 60 degree washes that all manufacturers say we should do regularly ( Washing machine manufacturers now recommend a maintenance wash once a month ) to keep the washing machine clean inside. However, it would need sophisticated techniques to overcome the problems mentioned that caused the demise of the hot valve in the first place and involve wasting the first draw of water which would need to be pumped out (there’s no room to store it for later use in the rinses).

if it is both hot and cold then the people who are concerned about the length of a cycle are you just using the cold fill if you are then the machine will take time to heat the water and thus make the cycle longer