Some thoughts.
Possibility of nEMP.
1. A nuclear weapon is required. There are not too many around in the megatonne range required for nEMP.
2. A specific weapon, high altitude burst, is required. Very, very few around, if any at all. Only ever tested by the US and USSR, 1958-1962.
3. A launch platform has to be used that reaches very high in the atmosphere. Very few people have the ability to do this.
4. Some sort of aggression is required. e.g. I really can't see the US attacking the UK or vice versa.
5. Should you have all the above, a fair number of weapons (perhaps 10-15) are required to cover the US with confidence (1/2 to 2/3 of missile based weapons are expected to fail, either delivery or completion.
Putting all that together as a package reveals that those with the availability and inclination currently number zero.
Outcome of nEMP:
1. Whilst Nemp certainly exists its effects are debatable. From the twenty one exoatmospheric tests the only damage I can find are some streetlights and a telephone exchange from 'Starfish Prime' and a rumored fire in the power plant in the city of Karaganda from Soviet test 184.
2. Many utilities are well protected these days from surges because of the threat of natural energy pulses.
3. Much theoretically vulnerable equipment will not be exposed to the effects of nEMP as it is indoors, unconnected, physically too small and so on.
4. Even if equipment was lost through nEMP much of it is not vital to life or the continuance of society or culture. We will easily carry on without television for example.
Putting all that together as a package reveals that the chance of being affected by an nEMP is vanishingly small.
Classic risk calculation deems that risk = likelihood X severity. In this case zero times vanishingly small.
In the UK ten people die every day on our roads and thousands are seriously injured.
My thesis is that the effort, time and cost one puts into preparing for the outcome of an nEMP would much better be put into, say, a yearly safe driving course or a more effective anti-sun cream to avoid skin cancer.
I agree that the UK and US probably won't be trading shots at each other. That's about all I can agree with here.
Several countries, unfortunately, can deliver a nuclear device high enough to cause problems. It doesn't take dozens of weapons, one high up can and will hurt the country very badly. EMP has 3 components to it, and the first (E1) is also the fastest and most damaging. It has been simulated and tested on cars and other modern day conveniences, and it is a serious threat. The E3 component is similar to that caused by CME's from the sun.
Utility companies are very unprepared for this, a quick google search can bring many recent studies showing this. We are very unprepared in the US, maybe in the UK things are different. Current estimates range from 18 months to several years to get power generation back in the US.
The Soviet tests in Kazakhstan took them completely by surprise, the damage was much, much worse than expected. If you can't find reports of serious damage, then you haven't looked very hard at all. I suggest you try google.
The E1 component can and will take out a lot of very important things in life. It's not living without television, it's living without electricity. No medicine, refrigeration, fuel, heat. The list goes on and on. Our society relies on electricity for all of these things.
Either EMP or CME will have catastrophic effects unless we get our power grid protected, and we are a long way from that.