Where in the UK is area code 0207 511?

There is no STD code '0207' in England.

However, 020 is the area code for London, always followed by an eight-digit local number; the "7" following "020" is in fact part of the eight-digit 'local' number.

Occasionally people still display numbers with the spaces in the wrong place, making it look as though the code is 0207, 0208, 0203, etc...

More specifically, most (but not all) UK phone numbers beginning (020) 7xxx xxxx are likely to be in Central London. http://www.magsys.co.uk/telecom/codelook.asp features a tool that may help you confirm a more specific location.

Note that there is no longer any official distinction between inner and outer London in phone numbers - it's quite possible for an (020) 7 number to be used in the outer suburbs, or for (020) 8 numbers to be used in Central London.