Ever since The Man replaced the Victorian water mains around here the water pressure went all to pot and the difference between being frozen utterly to DETH under the shower and being boiled alive is about 0.00001 degrees of cold tap rotation.
If your shower mixer is getting mains-pressure cold on one side, and gravity-pressure HW from a loft tank on t'other side, this is never going to end well.
Even a thermostatic mixer will struggle to deal with this.
It needs re-plumbed.
Either feed both sides from the loft header tanks ( with a pump for adder vvvroom );
Or upgrade the DHW cylinder to mains pressure ( megaflo and the like ) so both sides are at mains pressure.
Oh, and as mentioned already, you could lose the HW cylinder totally, and go with a combi heat-on-demand thing, which also gives mains pressure HW.