Has the plumber turned on the water to the bathroom properly, are you able to get a decent flow\pressure from the taps in the bathroom sink?
Secondly does the flow rate improve if you hold the shower head near the shower tray?
If your getting pressure to the taps, and you have a combi boiler then your water is mains pressurised, and as such would point towards a fault with the the pipe work.
Its unlikely to be the thermostatic valve, to me I think the likely causes are:
1. The pipes are not 3/4 inch to \ from the valve
2. A service valve has been half closed
3. The valve is faulty or incorrectly configured with a flow restricter in place.
The first steps in troubleshooting, are get a pressure gauge and measure the pressure coming out of the shower heat, which is just unscrewing the shower head an attaching the gaugue, this will show if the pressure is still at 2 bar.
Next its a case of switching off the mains water and removing the valve, and checking the valve hasnt got a flow restricter in place, and the pipes going to it are 3/4 ".
Lastly its a case of looking at the pressure and flow rate of each pipe to understand if the fault is prior to the water getting to the valve, and can be measured by turning on the water and counting how long it takes to fill a litre of water.
From here if it takes longer than the sink tap, its a case of tracing the pipe work back, of failing that its a case of looking from the valve, back up to the shower head.
In any case I would expect this level of troubleshooting is for the plumber to sort out, as this sounds like poor workmanship.
I havent heard back from you, please let me know if I can be of any further assistance.
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