You say you work full time. Does your work provide any sort of social activity? If so, then you could get involved with these and hope to meet new people? Even if there isn't anything official, what's stopping you going out with your work friends?(that's assuming they are the kind of people you would want to socialise with!)
Other than that, and relying on old friends, I am not too sure what to suggest. London can be a very lonely city at times.