Medicine, realistic questions

Can you afford to pay international fees?

I think you need to really look at the US and UK for medicine completely separately as they are totally different systems.
For the UK:
Yes your school grades are important. However different medical schools will have different methodologies for shortlisting applicants and so may emphasise different parts of your application.
GCSEs are a UK qualification. If you are not in a UK style education system you won't be expected to have taken them - this will be equivalent to whatever qualification(s) you do in Italy between ages 14-16. Medical schools will be familiar with major international education systems and will interpret your results in the context of your country's qualifications and education system. Same applies to e.g. A-levels, IB etc (which are 16-18 age education here).
The UCAT is an admissions test. It's taken as part of the application process, and often forms part of the shortlisting methodology of medical schools to determine who to call to interview. You can find a lot of information about this online but it's very much a "general" test and not focusing on subject specific knowledge (although you can and should prepare for it as the specific question formats are quite particular). This would be required for all applicants if they require it as part of their admissions requirements - including international applicants.
In the UK, assuming your aim is to practice medicine in the UK, where you study medicine does not matter. The GMC accredits all medical degrees to the same standards, and considers all medical schools equal. Equally, the NHS is the only provider of graduate medical training posts (jobs) for newly graduated doctors and takes the same position - and to ensure there is no bias, they blind recruiters from your medical school data so they cannot be biased based on where you studied medicine.
In general medical schools in the UK are very transparent about their entry requirements and shortlisting methodology and you can find all this information on their websites
Note however the UK government imposes a quota on the maximum number medical students each medical school can take, and each also has a smaller quota of international students they can take. Therefore it is very competitive for international students (as well as expensive).
For the US, medicine is a postgraduate degree only so you would need to complete an undergraduate degree in something else somewhere else first anyway. Also the immigration/visa rules in the US as well as other factors strongly favour US medics (and green card holders) when it comes to applying to jobs as I understand, and so international students tend to be limited to less popular specialties or less desirable (often more rural or deprived) regions as they can't successfully apply to competitive specialties in popular (often metropolitan) regions.

Thank you so much for your response! I really appreciate the advice and insights you’ve shared.

Do you have any tips on how I can stand out in such a competitive field? I know that getting into medical school abroad is extremely challenging, so I’m looking for ways to strengthen my curriculum or application. Is there anything specific I could do to make myself a more competitive candidate?

I think it's less about standing out and more about ticking boxes very well. Aim to vet a very good UCAT score, maximise other areas of your application based on how they shortlist and apply tactically to the medical schools whose shortlisting methodology fits your profile best. I believe typically offers are principally and in many cases wholly made on the basis of the interview so once you get invited to interview its more about preparing for that - many medical schools use an MMI format for example which has a very structured format that you can make a point of preparing for very well I guess?