Hi MelItaly,
A Phd in Britain usually takes 3-4 years, I believe most people take 4 to finish the write-up.
In Britain Degrees are graded as a First 1, An Upper Second 2:1, Lower Second 2 and a Third 3. 1st in the best and 3rd is the lowest before a fail. Most Universities need a First or Upper Second. I'm not sure how Italian Universities work but a 1st is 70% or more and a 2:1 is 60-70% of pass marks.
Applications vary according to the University you are applying to, some just require a CV and covering letter, others have specific application forms to fill in, they will usually tell you when you enquire about applying. Look at findaphd.com to look through some phd's available.
Hope this helps!
Feel free to ask anything else.
Caro