The symtems you describe are consistent with a loss of the netural
connection somewhere in the circuit.
check all connections at shower, double pole switch, rcd, YOU have got
one??, and in the CU itsself.
When you check the voltage always check between phase & netural, not
phase & earth as you are doing at the moment.
ALWAYS SWITCH OFF THE POWER before YOU POKE YOU FINGER INTO LIVE