I am a green card holder. I am going to France, Spain and Italy with my US citizen friends. In addition to my Schengen visa, do I need a separate visa for London?
As a green card holder and Filipino citizen, you will indeed need 2 visas.
-One for your trip within the Schengen area (France, Spain, Italy). The consulate you apply to will be the one representing the country which is your main destination (most days spent) out of these three.
-And another visa from the UK so that you may visit London.
The UK is not in the Schengen area which includes most of Europe. This means that there is passport control and in addition to the Schengen visa you got for France, Spain and Italy you will need one for the UK.
While not official, this website seemed to spell things out quite clearly: https://www.tripzilla.ph/uk-tourist-visa-philippine-passport-holders/748