Interesting that you identify yourself as a Brit and yet you want a Canadian passport. Nonetheless here is the official site for passport applications.
http://www.pptc.gc.ca/abroad/index.aspx?lang=e
Your Canadian Birth Certificate is your guarantee to a Canadian Passport. Essentially any professional can be a guarantor like a dentist, or bank officer or pharmasist etc. There is a link that will show you who can be one. Follow the links to download the form, fill it out and mail it in. No need to be near a consulate or High Commission. (PS we are part of the Commonwealth of Nations - we have High Commisions)
I am assuming your children were not born in Canada. In that case they have to go through the immigration process to become Canadian. They cannot become automatic Canadians because they are over 18 yrs. You can sponsor your children upto 22 yrs of age under special conditions. They have to be full-time students and fully supported financially by you. But more importantly they have to become residents of Canada for 3 years in a total of 5 years to qualify. That is called the Permenant Recidency requirement. Here is the official site to sponsor family members.
http://www.cic.gc.ca/english/sponsor/index.html
Finally, beware, there are many sites and people who claim to help people immigrate to Canada. These other sites tend to create unnecessary confusion. Obviously they have business to gain. Do not get scammed. They have no more influence on the decisions than you do. There is no Government sponsored agencies. Immigration process is fairly simple and designed to be handled by individuals like you. Follow the official site I listed above and you'll be in good hands.