Ehhhh... The early bombs were pretty inefficient, payload to explosion wise. Little Boy only had about 7-8% effiencies, and Fat Man topped out at 15-18%. It would be much better to actually use the '80s era hydrogen bomb since it's hitting the end of it's shelf life already and you don't have to redevelop the weapons integration.
I think watching the hospital and hospice system buckle might wake some people up. US and UK causalities were both shipped back to London and environs for long term treatment, due to lack of working hospitals in France, so the constant influx of the wounded downtimers would in my mind quickly add a jolt of the new reality back into the Isles.