MonthlyRegent's Park (The Hub)freeGreat alone
Baker Street Irregular Astronomers
What is overhead changes month to month, and the same people are there to point at it.
What it is
Amateur astronomers set up telescopes on a park terrace in central London and let whoever turns up look through them.
When
One evening a month on the terrace at The Hub in Regent's Park. Dates on their site.
Where
Regent's Park (The Hub)
Who it's for
People who have never looked through a telescope and would quite like to.
First time
Free, no membership, and you do not need a telescope. Dress for standing still outdoors after dark.