What about when you arrive back in the UK so you need to inform them about the extra countries you've been to ?
So you're saying you could tour Europe and as long as you give them your departure airport and arrive back on the date and to the airport you told them about you don't need to give any extra information at all ?

That was the exact question I have asked when they visited me. Their answer was "You can, but you don't have to" (inform us)

If someone travels to for example Turkey (could be anywhere) and stays for 90 days could they within that 90 days visit another country and then return to Turkey ? 

You obviously need to give details to the UK police that your travelling to Turkey and intend to stay for 90 days but do you need to give them details if you visit another country for a few days if you still return from Turkey on your expected date ?

Would you need to even tell them you visited another country or even 2 other countries when you returned ?