GCHQ is part of the intelligence community....
And we already know that GCHQ uses American intelligence programs, like PRISM, to "get around" their privacy laws, thanks to Snowden.
Most likely, our own political leaning intelligence guys, weren't worried about breaking any laws to spy on Trump's team, but if they were, how convenient that the Brit's owe them a favor.
Definitely makes sense why Obama changed the rules on who had access to this intel before he left office. It was to guarantee that all illegally "unmasked names" made it to the propaganda media.