Very low water pressure on cold for shower

Hi there

any advise on this please? I have a coachman Avocet/Arcadia 2020, and when using the shower the pressure is very good when selecting hot, but moving tap to cold the water pressure drops of to almost nothing just a dribble sometimes and sometimes less than 50% pressure compared to the hot. Meaning scolding showers! 
I don’t get this problem on either the washroom basin tap or the kitchen sink tap. Pressure is maintained on either hot or cold all the time. But not on the shower?? . System has been bled well,  Any ideas to what’s going on ? 
Thanks.