A pre-emptive nuclear strike on the EU and UK, in order to be successful would need to destroy all enemy military targets, including nuclear capability, communications, airfields, bases - everything.
Many of these things are in cities, or nearby cities - which means that in the exchange these cities would be targeted and destroyed.
In a nuclear explosion, most of the energy of the bomb is released as pure heat, the nuclear tests of the 1950s and beyond were all carried out in deserts and in the atmosphere where nothing flammable really exists. If you "nuke" a large city with a modern nuclear weapon, a city full of materials, chemicals, plastics, woods, factories, etc - it releases so much debris into the atmosphere. If you do this to many cities at once (for example all the EU and the UK, a lot of cities) the result is that so much stuff ends up in the atmosphere, the temperature of the planet - the whole northern hemisphere drops so much, that;
The temperature in the northern hemisphere drops 10-20 degrees in the first 6-12 months, due to the smoke in the atmosphere.
All crops fail and can't be re-grown, no food. (this is happening to most of the planet)
The atmosphere is full of radioactive dust, water is poisoned and remains poisoned for a long time.
Due to reliance on food-aid, nearly all 3rd world countries perish, due to America having no crops to provide aid.
Basically the effect on the climate, of unleashing that many nuclear weapons upon that many cities in one go - wouldn't be that far off an extinction level event, nobody needs to retaliate - and this is why in the 1980s, the scientific community including Russian and American scientists all agreed, that a pre-emptive strike is suicide, even if the other side doesn't get a single warhead away.
