Safe to use temporary water shut-off with mixer-tap shower?

[Autechre]

Is it safe to use one of these screw-in shut-off valves at the base of my shower unit, to save a bit of water/money? They leave a trickle of water, so the water isn't completely stopped from flowing.

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From reading the blurb, it would seem that practically every shower is incompatible and I wondered why?

https://www.savewatersavemoney.co.uk/products/view/38510/Flow-Saver.html

"FlowSaver is NOT suitable for Electric Showers, Digital Showers or those with multiple jets. FlowSaver is also NOT suitable for showers run by a mixer tap with a combined hot and cold water control."

This is my shower, which is fed by a combi boiler and mains water. Is this an example of a "mixer tap with a combined hot and cold water control"?

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Cheers.

 

[PrenticeBoyofDerry]

By the description it would seem they are only suitable for single controlled outlets, so would it not be simple just to turn the water flow down on the tap, as required? Seems a pointless piece of kit!

 

[Autechre]

The handle's a bit stiff for my elderly parents, so a quick off and on again could save a nice amount of water.

I'm not wedded to the idea, but I found a shower-head with the shut-off built-in, which I'd prefer to buy, but it's more expensive than this obviously, so I thought I should check-out the technology first.

Any idea what the problem could be?

 

[PrenticeBoyofDerry]

Any idea what the problem could be?

With regards to stiff tap?
Probably a build up of lime scale, would need striping down and cleaning.

 

[Autechre]

Thanks, but I meant what the problem was with this device.