Dear Zeshan,
I second carbonpaper on that, there is no need to go to consultants now a days as everything is available on internet.
If you go to
www.cic.gc.ca and follow the category you want to apply under. Forms alongwith guides as to how and what to fill in the forms is all available there.
First you have to fill in all the forms and send them to Centralised Intake Office, Sydney, Canada alongwith application fee.
Once approved, they'll send you an email to submit all the updated forms again with all supporting documents listed in the checklist. You will send all this to your local visa office, which is London in your case and wait for further correspondances from them.
If you search all in this forum, you'll find every single detail on this forum as well, kind of step by step guide.