My mother tongue is Cantonese too, but i'm buggered if i know how to read or write it! I moved to the UK when i was young, and so English is more familiar to me than Cantonese.
Its not bad or anything - I find that people who moved here at about the same age or older often don't have as good a grasp on cantonese as I do, but its far from being the same calibur as those who grew up in HK.
For instance, I don't know/understand cantonese slang as a general rule, and although I can watch chinese films and understand most of it [including some jokes], most of the slang goes over my head and I just can't appreciate it in the same way as if I was a HK "local". Also, I don't understand formal language either, for instance, i can only just about grasp the gist of a news report on TV. Its all because my parents wouldn't say that kinda thing to me in cantonese - they're too old to be using slang all the time, and of course there's no need for formal language when speaking to each other.
I have to say I prefer English because I find it richer - I can express so much more in English than I could ever do in Cantonese. My vocabulary is larger, and i know more slang, proverbs etc...
I do wish i was better with cantonese though - at a university with LOADS of cantonese speaking chinese students, i only have one friend i can speak to in cantonese! They tend to stick together a lot - I'm just too "white" for the majority them and so i was never integrated into their group.
I'm scared that one day I'll just never get to practice it and i'll become less and less fluent in it!
My friends find it hilarious when I'm on the phone to my parents though, I switch between Cantonese and English, as i apparently have a chinese fresh-off-the-boat accent to my english when its mixed up in a cantonese sentance, haha! I'm not entirely convinced that I do though...
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Other than you two share the same beguinning and ending letters 
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