Should GM crops be produced in the UK too? Why/ Why not?

If you believe it is good to grow them in the US or anywhere else, of course they should, Why should the rest of the world benefit and the UK not.
Of course if you do not believe they benefit mankind then they should be grown nowhere.
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Crops have been genetically modified for about 100,000 years.  Have you seen what corn looked like when Cortez first ran across it? No farmer would plant it today. Have you eaten seedless watermellons? They are genetically modified.  Tulips?  Holland had a flurry of genetically modification in the 1700's. some were good, some were not.
Horses? All the best race horses are genetically modified.
Eggs? Chickens are geneticlly modified to produce more and ao an ad infinitem.

I'm not worried about eating the produce. I am very worried about what can happen in a few generations of the plant. Nobody knows what will happen. The pollen gets blown around and can interact with other species, several are used to create the new plant.
What if in say five years, the plants just don't produce working seeds? We would all get hungry very quickly and there would be no going back. Everything would be contaminated.
If you're happy to risk that in America then carry on, but if any experimental seed gets introduced to the UK I'll be pleased to join one of the groups that cut through the fences and burn it.
A recent very expensive and heavily protected test in the UK showed no benefit from whatever the mutant seed was supposed to protect itself from.
Breed new strains of plants by all means, select the strong ones and breed some more, naturally,  but don't mess about with things that are not fully understood when it could risk the whole world's food supply if it goes wrong.

You might also consider that mutations happen naturally in just about every complex organism over the organism's lifetime. Over a large crop, the total number of mutations is fairly large. Most mutations aren't passed on and, of those that are, only a few are significantly beneficial to that species.

If GM crops are to be prohibited, should natural (random, unpredictable) mutations also be prohibited? If not, then why is GM a problem?

Gm involves the physical insertion of dna from unrelated species. These mutations could never occur naturally as the two species could never interbreed. As you said if it was too much of a mutation it would not be passed on, but that is nature's way. If the dna has been artificially altered with the plan that the mutation is passed on then it will be. Nature will also be performing it's own mutations at the same time, but now instead of a normal progression there is a completely altered new organism, one that couldn't have occurred naturally -if it could then there's no point messing around with the dna, just breed it normally. This new organism could pollinate other species, nobody knows. Mutations that occur could affect the number of generations possible from this state until it goes sterile. Again, nobody knows.
Testing this on the world's food supply is not a good idea. They could spend a few decades testing in a totally sealed environment but instead they plant a few fields of the stuff allowing free access to insects and the wind so they have no control over any cross pollination that may occur. It sounds dangerous and has also been proven unnecessary as field trials were affected by a mutating species of the insect the crop was supposed to protect itself from.

This new organism could pollinate other species, nobody knows.

Nor does anybody know what some natural mutation might do, and they achieve pollination all the time. Every organic toxin known came from natural mutations. Who knows? What is the practical difference between a natural mutation and "one that couldn't have occurred naturally"? Is there anything that shows that one is more dangerous?

I submit that naturally occurring mutations are more dangerous because they are random and unpredictable. GM, however, has plenty of experimentation before general release; so there is a degree of experience with any specific case. The resulting plant has been observed to test what results.

And there is something of a built-in safeguard in that enormous potential liabilities tend to put brakes on risky releases. Few corporations are interested in multi-billion-dollar/Euro lawsuits, so a lot of testing goes on. The same can't be said for natural mutations.

Since we can't predict effects of natural mutations, why does it matter if GM gives results that can't happen naturally?

If some possibly dangerous mutations already occur naturally, why double the risk by creating our own?
The testing doesn't seem to be isolated enough. Planting a field of the stuff outdoors let's it come into contact with everything.
I believe they decided that insects could adapt faster than the crop would grow so there was no benefit as they soon developed a taste for the new crop. If you follow that line of reasoning we will soon have insects that can adapt almost instantly to any change. Even spraying for them wouldn't work, they'd learn to eat it. Just a kind of arms race and one that we would lose.