I have no idea what the "city&guilds London diploma" entails, but it doesn't sound like it involves much mathematics and science. These disciplines are what you must excel in to have a reasonable chance at an electrical engineering degree.
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"Don't disturb my circles." translation of Archimedes last words
I HAVE NEVER HEARD OF CITY AND GUIDES BEFORE YOUR POSTING. I AM NOT QUALIFIED TO MAKE A RECOMMENDATION FOR OR WARNING AGAINST THEM.
You seem to be asking about http://www.cityandguilds.com/uk-home.html. They seem to be a vocational school. Based upon that the answers are yes, not, kind-of and it depends upon you and what you want.
The YES part: Any good engineer is an engineer. School is important, the piece of paper from the school is important, but what makes you an engineer is how you think and what you want to do. Thus, any technical training and any technical experience helps you become a better engineer.
The NO part: A degree program in engineering requires a lot of advanced math and science along with some language, social studies, etc. The degree helps to prepare you for design, development, failure analysis, evaluation of options, risk control, leadership, etc. A very large portion of a degree program will not be covered in a vocational program.
The KIND-OF part: A lot of the material in a degree program is taught with little or no hands-on experience. Some people can learn a lot from books without ever needing to see how all the mumbo-jumbo facts relate to the real world. But, during the first day of a degree program they will probably tell you that "Engineering is the application of math and science to fulfill the needs of people". There is a bit of a conflict here. The degree program is easiest for the people that are not driven to do things but you are training to be able to do things. Thus, the good news is that vocational experience can be a narrow-topic-range of useful experience. The bad news is that while you gain vocational experience you are getting older, forgetting what you learned in your other school classes and running the risk of getting married, having kids, etc. (you often don't have time for a degree program after you start family life).
The IT DEPENDS ON YOU part: Read all comments from CR4 posters, get comments and suggestions from others, prepare yourself as best as possible to make this decision and then make the decision that is best for you.
Side note: most advisors will tell you that if you want a degree you should go directly for that degree. The more time required to complete your path the lower the chances that you will get there.
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Few things limit our potential as much as knowing answers and setting aside questions.
"The YES part: Any good engineer is an engineer. School is important, the
piece of paper from the school is important, but what makes you an
engineer is how you think and what you want to do. Thus, any technical
training and any technical experience helps you become a better
engineer."
The above is not applicable to Canada. Paper is first, school is first. then you can qualify to be an engineer. No matter what your how you think and what experience you have, especially if you have dark color skin, you will have a lot of obstacle to become one of the engineers.