You have approx 1/2 bar of pressure from your cistern so this will be less than your mains supply
You may be better purchasing a shower with an integral pump which may involve upgrading your electric supply cable.
My thoughts are that you should look into why your mains pressure is so poor.
Could be that stoptap is knackered, seized half on, incorrectly fitted with arrow pointed the wrong way, boundary stoptap not fully on, shared main.
Might be worth asking water supplier to test supply, you never know they may fit you a new supply for free