Estimate correlation in Python
You could create a second label for your usernames according to whether they contain london or not (pseudocode below):
for idx, username in df['Usernames']:
    if 'London' in username:
        df['London'].iloc[idx] = 1
    else:
        df['London'].iloc[idx] = 0

Consequently given you want to go with correlation and that you are comparing binary variables, the metric to go with is Pearson correlation coefficient (pseudocode below):
import scipy.stats.pearsonr as rho
corr = rho(df['labels'], df['London'])