If the boiler engineer disconnected the outlet pipe on boiler to prove thats ok first thing to do,
Could be a inline filter somewhere thats part blocked they do put then in boilers as well
or a stopcock or isolator not turned on fully.
Depends what shower unit and finishing parts are to be fitted it may look ok when done/adjusted with fittings talk to person who fitted pipework to advise you.
Turn rad valves off drain rad then open each valve and see if there is a flow of water from each one with water in system if not than will mean a blockage.
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