It's incredibly rare that warrants are issued at all; since the late 18th century, it's a Speaker's Warrant that's used (and enforced by the Serjeant-at-Arms of the House; prior to that no warrant was needed at all). They are formally issued by the House, and not Parliament as a whole.

Non-subjects can be compelled, most recently Rupert Murdoch in 2011 (wrt the News International wiretapping scandal), which along with his son James was AFAIK the last time a Speaker's Warrant to summon witnesses was issued.