Since you are in Canada mate, you can't really pop into your local AFCO (armed forces career office), so the next best thing would be the online AFCO. Go to the british army website, and follow the relevant links. You can send them a question, and they'll get back to you. They are generally rather vague with their answers I find, but it's worth a go
I personally thought flat feet was a no go, but I don't know if it is these days. Perhaps it would be fine with a tank regt, but I can't say. Good luck!