Yes, any person who is NOT a British National needs a passport and in some cases a visa also.
Yes they do, just like they need a passport to enter any other country.
In theory, you don't need a passport if you are Italian but check with the British Embassy in your own country because the rules are complicated.
Yes they do.
passport
No, he needn't.
no
Yes
A valid passport is required
Yes they do.
No. All EU countries and Switzerland, can travel, work or just visit in the UK without a visa.
Yes you do. you need a passport to fly on a plane
Although both Greece and UK are in the European Union you still have to show an up to date passport in and out of both countries.