It’s still quite a lot of power for AM (amplitude modulation) broadcast on any band.

If Radio 4 was on shortwave with 500kW transmitters, I probably would have been able to listen in on my radio at home in Australia, no UK-located KiwiSDR required.

Japan used to have a 500kW transmitter on 774kHz (mediumwave/AM broadcast band) for NHK Radio 2, and when the local broadcaster on 774 kHz went off air at midnight for scheduled maintenance, I could pick it up on my car’s stereo quite easily.