So not a fixed 6.5kW load as per original OP, but one that can be terminated probably for 1ph 2.1kW element, which would account for it being ok (Not tripping or any thermal damage) on a 13A plug/socket.It's most likely been converted from 6.5kw heater to 2.1kw heater by the supplier, either from new or as used.
And at 6.5kW could be 3ph??