Your Mind Makes It Real: One of Captain Britain's powers is that (when he is playing the Super Hero, at least), people always see him as they believe him to be. So the Scottish see and hear him as a Scotsman, the Welsh see and hear him as a Welshman, the Americans hear him with a horribly stereotypical Cockney accent, and so on.
That may have been one of his powers in his own series previous to Excalibur (which I didn't read and can't speak to), but it wasn't one during or subsequent to his stint on Excalibur, and isn't listed as one of his powers on Wikipedia or Marvel's own website. Also, "Your Mind Makes It Real" is when someone dies or is injured in a dream or computer simulation and the death or injury carries over to real life, it's not when believing something affects your perceptions.
Removed:
That may have been one of his powers in his own series previous to Excalibur (which I didn't read and can't speak to), but it wasn't one during or subsequent to his stint on Excalibur, and isn't listed as one of his powers on Wikipedia or Marvel's own website. Also, "Your Mind Makes It Real" is when someone dies or is injured in a dream or computer simulation and the death or injury carries over to real life, it's not when believing something affects your perceptions.