I saw this last night and thought it would cover me for flying out Monday and returning Wednesday (can't quite make the test fit within 48 hours of my return flight) but ended up reading it so many times I'm now worried it only applies to flights before the new rules come into effect. The sections above it are talking about tests within 3 days but we know that from 4am on 7 Dec it is 2 days.
Yep, this is what I was thinking too. The main header of the page says:
Changes to pre-departure testing from 4am, Tuesday 7 December 2021
From 4am, Tuesday 7 December all people aged 12 years and over must take a PCR or LFD COVID-19 test before they travel to England from abroad.
You must take the test within the 2 days before you travel to England.
This will apply whether you qualify as fully vaccinated or not.
And the 3 day thing was in place before these changes.
However, (admittedly not Italy), but Eurotunnel says for 'day trips' it the period is 3 days (in the FAQ):
https://www.eurotunnel.com/uk/travelling-with-us/latest/covid-19/Oh it's so confusing!