Basically, the instructor who is going to train you on the type-rating is not allowed to sign that rating on your italian licence. That is because he would be a UK CAA authorised TRI with no rights to fill anything on your italian (JAA) licence.
So, you have two possibilities:
1. ask the UK CAA to issue a licence based on your italian one.
2. do the type rating, bring all the papers to the Italian CAA and ask them to approve it.
It should all work easily, according to the JAA regulations, but we are talking about the italian ENAC, and I am afraid this is going to be really tought!!
I guess the easiest way should be the first one, you could have some problems due to your italian "not fully JAR" licence, as it happened before to many pilots, but however you should try, the UK CAA is always very helpful.
I would rather not try the second option, I don't think anyone there at ENAC will never approve that!
JAR should be the same in all countries, but it's not as easy as it should, and being italian as well I guess you know what I am talking about!
Buona fortuna