It is not possible to design a low pressure tap with the features
desired.

They are for sale. If the mains pressure is high, 7 bar for e.g., there
may be problems, so a pressure reducing valve on the cold of that tap on a
low pressure mixer is cheaper and better than a pump on the hot.

Could you point me at a link for a mixer tap with hose attachment and
internal body mixing that is suitable for mixed gravity/mains operation. The
hose has to provide high pressure very hot water. This high pressure water
has to be at least 55C, as it is for dishwashing, not showering.

Christian.