Run your shower from a pumped supply from a tank in the loft. Or pay the money for a nice 25mm connection from the main. Sounds like yours is a lead pipe with the water equivalent of atherosclerosis.

I live in the UK. I am on a shared water supply with one neighbour. I have reduced water pressure when they use their water at the same time. I am finding it rather dangerous as the shower turns to scalding hot, then clicks off all together and then comes on again freezing cold when they stop using their water. I have been in touch with SW Water. They say this is a problem of having a shared supply and there is nothing that they can do about. This doesn't seem right or fair. I pay the same water rates as everyone else, surely I should have a supply that is constant. Has anyone experienced the same and can you tell me how you overcome the situation. Many thanks for any information you can give me.