The US has some very close allies for sharing intel, and it'd be a massive incident if it got out that such allied countries were spying on each other. It happens to some degree, but if stuff gets out, the White House & State can make heads roll, so you'd only see pretty routine spycraft occur between allies (counter-intel, rumor mill, feelers).
The trick is not to get caught... and if you do get caught, then blame it in on the Chinese/Russians/flavour-of-the-month :D
In all seriousness, the spying may not be as hardcore or blatant(!) as say US/China or US/Russia but they are not looking for the same kind of intel between US/UK. I wuold be very very surprised if there was not some intel gathering at some level.
