Range from the cheapest Fit a new plug socket £134 (UrbanVolt Electrical Ltd based in BR2) to the dearest Fit a new plug socket £220 (Bromley handyman based in BR2).
Comparing electricians estimates for a Fit a new plug socket gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes by posting your job to fit a plug socket to electricians in Greater London.

"Fit a new double plug socket"
BR2
"Change 20 plug sockets and 6 light switches "
BR3