So, as stated above -
The UK marriage certificate, available from the GRO, would have the names of both parties' fathers, as well as the fathers' occupations (as stated to the registrar). Also, it will state who the witnesses were, and where they married. Possibly, if their place of marriage still exists, you would be able to contact that place and ask if any other records exist (e.g., membership transfer records). Of course, if they married in a registry hall, there would be no additional info
GRO records are available online, pretty inexpensive, and pretty quick to return.
Italy is a tough research place - most records are not online. Plus you can't be sure exactly where in (around?) Florence that he hailed from. And I don't think very many Italians are testing, so the people you match are often in the same situation.
As Peggy Deras stated above - Y-DNA testing might help. It looks like Umberto could have living direct-male-line descendants. But to be really helpful, someone also from the direct male line needs to have tested.