Why are Italian sausages so different from those in the UK?

I agree with your assessment of the sausages between the UK and Italy, getting decent sausages in the UK is harder, but not impossible. Commercial sausages in the UK tend to be very finely ground, and have rusk or some sort of meal to act as a filler and absorb juices as they are cooked. UK sausages tend to be flavored by herbs, or have apple in them. Italian sausages are generally more coarsely ground so you see distinct pieces of meat and fat throughout, they also have less fillers so they shed more fat when cooked.

In the UK you get what you pay for with sausages, if you get the cheap store brand stuff you'll get sad tubes of grey paste which will split the second they experience direct heat and spray so much fat the entire barbecue starts on fire. If you shell out a bit more you will get a better product. My advice is to find a good butcher, many make their own sausages in store and they usually have good variety. My local one has quite a few and one is an actual authentic Italian style sausage which is excellent, you mileage may vary but it's worth shopping around.

Not true answer, but long for a comment:

In US, most sausages tend to be ground multiple times and a course grind in the US is what many others might call a fine grind, and I think most UK sausage techniques are close to US. Spices and varieties vary, but techniques are similar. What you want is probably more of a chopped meat rather than a ground meat. Biggest issue there might be getting that quality and still getting a reasonably even distribution of fat for fairly uniform cooking.

A question, the sausages you liked in Italy, was the meat possibly cured or smoked before stuffing the sausage? This might well be an option to reach a chunkier, more meat consistency along with chopping rather than grinding. Most "Italian" sausage I have seen tends to be fresh, while in Italy there may be more of a tendency towards curing as well. Curing salts and such often have the tendency to firm up the meat. A selection of the source meat can also effect. US sausage tends to often use lean meat with added fat, but if a more marbled meat with less added fat is used you may be able to grind/mix less and reach closer to your target.

I would suggest researching both sausage making and charcuterie techniques and experimenting. For books you may need to stray from US or UK published and try for translations from French, Italian, maybe Spanish. As for pork, sounds like may need a weekend drive to the country with an ice chest.

ETA, yes, from your picture, I would class your UK sausage as a fine grind, probably through the grinder twice and a third time for stuffing. The pictured Italian ones are more of a chop and mix, or single very course grind and then stuffed with a press stuffer, not with another trip through the grinder.

Can't comment on the state of sausage in the UK in particular. However, I can comment on sausage in general.

The term sausage if very broad. There are many different kinds of meat that can go into the sausage. They can be finally-ground, coarsely ground, or even cut into chunks with a knife. Sausage can vary in how much fat, offal, or non-meat fillers are added.

Please consider visiting a Polish deli or butcher in the UK. Polish people love meat. There is a huge variety of sausage in Poland. It's not all great and you can find poor-quality stuff as well. However, the spectrum of quality is big and you are bound to find something that will satisfy your tastes if you check out Polish sausages.