I don't know if they have mockingbirds in England, but they sing all night if they feel so inclined during their breeding season (and possibly year round). If it repeats the short phrases of its song several times in a row and strings together distinctly different sounding bits, it's probably at least related to a mockingbird.

I've wished for a gun many times when the birds in the bushes outside my window start at in at 2am during the summer.