Shower: Water supply question

I replaced my old mixer shower with a Mira Sprint last weekend but it is showing Low Flow all the time It heats nothing which I assume is related saftey feature.
I did some measurements to the header tank and its about a metre. I just do not think I have the head.
My question is, as there is the mains feed pipe about 20cm away in the loft, can I source the shower from this? I'll put an isolating valve in as well, so I can limit the pressure if necessary.
Is that a plan? Thanks.
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You appear to have replaced a mixer shower, presumably fed by tanked gravity hot and cold feeds by an electric shower. The electric shower will be disappointing at best, but really has to be fed from the mains supply. They aren't really designed for gravity systems.
Christian.
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