In our old place in scotland the pressure certainly seemed excellent, down here in lancashire it doesnt seem as good, turning two taps on, or even the washing machine filling makes a reasonable difference to the water flow.
Since you have CH water via a tank I'd suggest a negative pressure shower pump for the loft. Will cost about £300 / £400 for a decent one and the price of a spur feed.
Location has to be near base of hw tank.
The pump could be plumbed to deliver tank fed hot and cold water to the loft which I'd assume also requires water services for a sink.
So since electric would only sort the shower, is likely to give problems on a 100A supply the pump is the perfect compromise and will never have issues with same time use of 2 x showers or shower / bath.
Also since the pump must feed via tanks the water pressure isn't an issue since I'd assume the existing shower is mains water.
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