Well, one of my favourite colleagues at work has a very dry sense of humour and might say something like that to me. It did take me a while to realise that it was just him trying to be funny by taking the p*ss, and not being rude to me. And so now, we go back and forth with him teasing me about my accent, and me telling him he can't be a pirate because he can't say "AARRRRR."

And he even gives me high fives now. So, I might be inclined to think it is possible that your colleague was joking and carried it on a bit too long, but really only you could know if that is sense of humour, and he is picking on you because he likes you, or if he is really trying to put you down. Either way, don't feel the need to change who you are. It's one thing if you are saying something that might potentially confuse people due to a different pronunciation or usage than they use here in the UK, but dude, everyone understands what awesome means!

Besides, tell him that Americans like it when English people say brilliant because it's an English sort of thing to say, therefore English people should find it fun and endearing that you say awesome.