If you want to read the novel for yourself look away now.
Basically Muslim terrorists hijack a BA airliner en route from west of Ireland and plow it into the caesium storage facility in "Freshpark" (ie Sellafield) nuclear power station. Five thousand foot fireball results in Caesium cloud drifting over north of England. Disguised as firemen, other terrorists gain entry to the stricken station and blow up the plutonium storage facility. Torrential rain. Northern England massively irradiated while radioactive cloud also devastates Holland and Germany.
British government move to Gibraltar, martial law declared. France closes borders. "Freshpark" fires contained after a month after massive loss of life. Northern England rendered permantly uninhabitalbe.
Britain is faced with loss of over twenty million of its population (even water has to be strictly rationed, reduced rations for the dying.) US and Canada offer to take millions but there are huge logistical problems in transporting these numbers across Atlantic before starvation and disease take their toll.
With genuinely no other option available, Britain invades and occupies Ireland. Over the next fifty years both countries cease to exist as we know them, southern England becoming a giant Gaza strip.
Unfortunately something nasty is going to happen one day; it is just a case of trying our best to avoid it. I believe the IRA summed this up with a statement along the lines of 'we only have to be lucky once, you have to be lucky all the time'.