You need to get the DNO to fit an isolator.
This involves the DNO pulling their fuse (hint) and updating anything
w.r.t. safety - for example old ceramic cutouts and cotton covered
meter cables or cast iron cutouts or old earthing or tar leaking lead
covered cables which are most definately not armoured.

You then need to decide if an instant mains pressure water heater or
stored water heater is more appropriate.
The latter can be a simple combi-cylinder - a CW tank stuck on top of
a HW tank in one foam insulated zeppelin, not mains pressure, but runs
off 2.5mm Flat Twin & Earth 3kW immersion heater.
If you want an instant water heater, you need a dedicated final
circuit with RCD protection (possible CU upgrade or second CU
fitting), plus consideration of expansion vessel or whether the
pipework can act as same if your water pressure is not sky high (ie,
preventing the pipework acting as the pressure vessel).

None of these are cheap, as in a few hundred pounds on materials alone
whatever you do.