There are some details not given by Pistonvalve such as frequency range, antenna type, and if it is also for transmission (so high RF power). But it occurred to me that many of the old valve radios with a "live chassis" (chassis connected to neutral so potentially live for any one of many faults) would couple the antenna and ground terminals using small high-voltage capacitors, and something similar might be useful here.
This low frequency isolation approach avoids the risk of high currents in an external ground rod needing heavy wire, or alternatively high touch potentials around the rod needing it to be shrouded, in the rare event of a fault. But as pointed out on the IET board discussion you need such capacitors to be safe if the worst-case happened and the local (in house) PME earth went high voltage, and also you don't want static from thunderstorms, etc, causing a build up of charge.
The attached diagram shows a possible way to do this.
Here the wires from the external metalwork (i.e. earth rod and antenna) should be considered as potentially at mains voltage w.r.t. the internal PME earth under fault conditions so probably wire with 1.5mm^2 conduit cable / earth wire (e.g Toolstation part number 67609) or similar that is adequately insulated.
Similarly the electronics must be in a box to prevent touching of the antenna/external earth conductors. Simplest is plastic, but if the radio will be grounded via the coax connector then a metal box will be fine if it is also grounded via that connector. The capacitors shown are 4.7nF class Y1 rated (e.g. RS 875-2421) and so safe even if there is full mains voltage across them. At 50Hz they have around 677k impedance so less than 1mA leakage, but at 100kHz that are down to around 339 ohms and probably much less then the antenna's impedance then.
The resistors are present to discharge any static build up. As for the capacitors they should be able to stand high voltages, hence using two 1M 1W resistors (e.g. RS 131-924) in series for the safety-critical route across the isolation barrier to have a rated peak overload voltage of 2kV (5 sec max). Resistance is not critical, but care in construction/placement should be observed to keep the two sides of the isolation apart by at least 8mm. Basically treat them as if mains voltage will be present.
Finally there is a balun (common mode choke) so reduce the noise from the PME side appearing on the antenna's signal. Shown in the schematic is a 1:1 version but google will bring up many designs (say search for "VK6YSF choking balun for HF bands"). If working at the low end of the HF band and an electrically short antenna then a 4:1 or even 9:1 balun might work better.