fist size insulation in the storage tank. Slow filling toilet. Does the toilet fill from the storage tank? Has been known for these clumps of insulation to block pipework. Another thing is numpty plumbers installing restrictive ballofix valves rather than full bore. Has the correct pressure inserts been fitted in the shower valve? Also check the bore of the shower hose.
Sounds like you could have got the same "performance" from a cheap shower booster valve these are less than £100 and easy diy fix.
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