Adapting 120VAC High Pass Filter to UK 240VAC
The "safety" of the circuit depends on the voltage rating of the capacitors. 450 V should be high enough for either 120 VAC or 250 VAC.

However, the corner frequency of the filter with the component values shown is 50 kHz, not 5.3 kHz.

If you use an additional isolation transformer, it will have a strong low-pass effect that may wipe out the signal you're looking for.