The one that gets me is people who signal right on roundabouts, but who are not going right, but straight on! (And then cut in front of you across lanes to make their exit.)

See Highway code section 186:
This section of the Highway Code explains the signals and positions required to exit a roundabout safely. Follow the rules and you will find maneuvering roundabouts to be much less stressful.

When taking the first exit (unless signs and markings indicate otherwise):
Signal left and approach the exit in the left hand lane
Keep to the left on the roundabout and signal left to leave

When taking an exit to the right or going full circle (unless signs or markings indicate otherwise):
Signal right and approach the exit in the right hand lane
Keep to the right on the roundabout until you need to change lanes to reach your exit
Signal left after you have passed the exit before the one you intend to take

When taking any intermediate exit (unless signs or markings indicate otherwise):
Select the appropriate lane on approach to the roundabout
Stay in the lane until you need to alter your lane to exit
Signal left after you have passed the exit before the one you want to take