DC Comics has two important British characters with powers, although both are magical/mystical in origin, and neither wear costumes.
Tim Hunter, the protagonist of The Books Of Magic, is a young British boy who is destined to become the world’s greatest mage. He wears glasses and has a pet owl, but believe it or not, the comic (originally written by Neil Gaiman) predated Harry Potter by several years.
Then there’s John Constantine, the antihero of the comic Hellblazer, and soon to be star of the upcoming movie Constantine, starring Keanu Reeves. John is a smug, smarmy, cynical con man and a right bastard, but he has been known to save the world more than once, trick the lords of Hell, and work alongside such heroes as Swamp Thing and Zatanna (his ex-lover).
Jenny Sparks was a British superheroine, a creation of Warren Ellis from his comics Stormwatch and The Authority (of which she was the founder and leader). The ageless Jenny had electrical powers and was considered “the spirit of the 20th Century,” since she was born on January 1st, 1900, and stopped aging at some point in her 20s. The closest she came to a costume was her usual look of a Union Jack T-shirt with a white suit over it. She sacrificed her life at the turn of the 21st Century after living 100 years of adventure, excitement, and heroism, saving the world from a Cthulhu-like “god.” (See Authority #9-12, reprinted in the “Under New Management” TPB.)
Finally, the purple-haired telepathic ninja Psylocke from the X-Men (Betsy Braddock) is British, and sister to Captain Britain, Brian Braddock. Of course, there’s a new female Captain Britan now, but who can keep up?