The hot water pressure relies on the water pressure from the cold water tank in your loft, whereas the cold water is (usually) from the mains. To see if this is your problem, set your mixer to full hot, then raise and lower your shower head. If the flow reduces as you raise the head, then you have an insufficient head of water above your shower head. If that's the case, your only answer is to raise your loft tank. Or spend loads of dosh on a power shower pump.