My main question was that once a semi bore isolator is installed somewhere does it mean that the flow after that isolator will be reduced?
Surely after a certain distance it builds up again, the way water through a damn maintains a heavy flow further downstream?
The flow rate along the pipe will be the same at all points along the pipe (unless you have a leak or a tee off to another outlet. Think about it, for the flow rate to vary along the pipe, the quantity of water going along it must be varying, and water isn't going to spontaneously appear or disappear.
Also, a half-bore valve (or any other fitting) does not restrict the flow rate - you can get the same flow rate by altering other parameters (within limits). What it does is introduce a restriction. When you put a flow of fluid through a restriction then your get a pressure difference (aka pressure drop) - even a length of pipe produces some pressure drop when the fluid is moving.
It is the overall pressure difference, in relation to the flow restrictions, which sets the flow rate. Your pressure difference is effectively that between your mains supply and atmosphere. The overall restriction is really the some off all the restrictions from the supply to where the water comes out into clear air - your mains supply pipe, main stopcock, all the internal pluming with lengths of pipe with fittings, various service/isolator valves, the heat exchange (assuming a combi), your shower valve, the shower hose, and the shower head. For an open vented DHW cylinder, you've got the head of water in the header tank as your overall pressure difference, and then all the plumbing and fittings between there and the nozzles on the shower head as your restrictions.
If you increase the total restriction, then the same pressure will give you less flow. So yes, adding a small orifice in a pipe run will reduce the flow rate. it doesn't do this my magically sensing and controlling the flow rate, it does it by increasing the resistance to flow - the flow rate will drop until the pressure drop in the larger resistance is reduced to match the available pressure.
Also, it doesn't matter (again within some limits, and for non-compressible fluids like water) where you put the restrictions. Putting a small bore service valve at the start of a long pipe run will create the same additional resistance as if you put it at the end of the pipe run.