If your purpose of going to

Canada is to apply for a new visa, then the only way you can come back is to get the visa. The first thing they will do is to collect your I94, hence you can't come back on a valid I94 within 30 days from Canada (this is called automatic visa revalidation).

If your purpose of going to Canada is travel, then the 30 days rule still applies.

You'd better get the visa done in Canada before you leave for England. I know for some countries, you have to be there for over a month, otherwise you are not allowed to apply for your visa three. But England should be fine, my friend went there once. He got his visa there. But to be safe, it is still better have it done in Canada. You will have a good chance there. Good luck.