Fluctuating Temperature/Low Pressure

I have a Triton T80 si which is plumbed into the same cold water supply as the bath (which I assume is the mains) but the temperature of the shower keeps fluctuating and the low pressure indicator comes on whenever any other tap is turned on. Can anybody recommend a better shower or solution to my problem?
Dai

You have a flow rate problem on the cold mains, or at least a section of it within your house.

First, check and, if necessary, clean the shower head (the part that sprays).

Next, check and, if necessary, clean the inlet filter on the shower.

If neither of those was the problem, then measure the cold water flow rate at the kitchen tap and compare it with the flow rate from the bath tap, and again with the flow rate from the shower when it's on the fully cold setting.