Thermostatic (blender) showers, need both hot & cold pressures to be equal, and combi-boilers are not known for giving enough flow for showers, even if you went to the trouble of fitting a cold header-tank in the loft just to feed your shower;-- you would just be plagued by chilly spells during showers, whenever another hot water outlet opened ( Kitchen sink or washing M/C).
A high-power electric shower, perhaps incorporating a pump, seems a reasonable way forward.
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