Shower Pump? Which Pressure?

I want to install a shower pump. There are loads on the market, all with different 'Bar' ratings. Can anyone advise me on what 'Bar' rating I need for a single 'over the bath' shower?
I think I have decided to get one of the Stuart Turner Showermate range, so its either a 1.4 Bar or a 1.8 Bar.
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