firstly for commercial you will need to have at least £5Million in Public Liability insurance, I know some on here will say you wont and you can get away with 1 million but most commercial work will require 5 million.
if your trad then there is little chance as because of health and safety you will need to be WFP, I'm not saying you cant get commercial being trad its just that your chances are limited.
f your phoning or calling on commercial buildings ask to speak to the facilities manager/manageress and tell him/her a little about yourself and what services you can offer him/her.
you will need to be flexable as a lot of commercial work is only doable out of hours or at weekends so let them know your prepared to be very flexable to suit the needs of their business.
best advice I can give is get your confidence built up doing domestic which is the bread and butter to all w/cs before you go looking for commercial work, you would be surprised how many w/cs pick up commercial via domestic.
good luck and happy window cleaning