Adal asked what happens around Covent Garden and Leicester Square.
There are very near to each other and both have candid possibilities, but Covent Garden more so.
Covent Garden has an old covered market which is great to look around. There are possibilities in there, but at one end you will find very good musicians playing for free in a sunken well area. I have made many photographs there, for example
Bebeto
http://www.pbase.com/clickaway/bebeto
ZHL String Quartet
http://www.pbase.com/clickaway/zhl
All around the market hall you will find street entertainers of various kinds, but do not forget to take the street due north of the market (Boots the Chemist is just on the right) and here you usually see about 4 living statues. I find that the presence of these 'statues' are also a great prompt for expressions of passers by.
Bebeto at Covent Garden market:
ZHL String Quartet at Covent Garden Market
Candid taken just north of the market at Covent Garden
The expression on these girls was sparked by seeing one of the living statues 'in action'...
Ray
PS - Adal, if you are planning to come over, perhaps we can meet up?