There is detailed guidance here on the restrictions that apply to different countries, including Italy:

https://www.gov.uk/government/publications/covid-19-specified-countries-and-areas/covid-19-specified-countries-and-areas-with-implications-for-returning-travellers-or-visitors-arriving-in-the-uk

There is a detailed map of the specific areas of Italy that are affected:

https://www.gov.uk/government/publications/covid-19-specified-countries-and-areas/covid-19-specified-countries-and-areas-with-implications-for-returning-travellers-or-visitors-arriving-in-the-uk

The full guidance for educational settings is here:

https://www.gov.uk/government/publications/guidance-to-educational-settings-about-covid-19/guidance-to-educational-settings-about-covid-19

The following restrictions apply:

Travellers returning from Category 1 areas: Travellers should self-isolate, even if asymptomatic, and call NHS 111 to inform of recent travel. [Category 1 areas are Wuhan/Hubei. Iran, Daegu/Cheongdo in the Republic of Korea and any Italian town under containment measures (as per map above)]

Travellers returning from Category 2 areas: Travellers do not need to undertake any special measures, but if they develop symptoms they should self-isolate and call NHS 111. [Category 2 areas now include northern Italy, defined as per the link above, along with the rest of China and Republic of Korea and a wider list of countries including Thailand and Singapore]

There are no restrictions for those returning from southern Italy.

Latest information

Updates on Coronavirus:  https://www.gov.uk/coronavirus

Public Health England blog:  https://publichealthmatters.blog.gov.uk/2020/01/23/wuhan-novel-coronavirus-what-you-need-to-know/