I drive that one a lot and I think I'd always get in the right hand lane to take the Horam exit (from the A22 northbound). As you say, people in the left hand lane there have a funny habit of going where you least expect them to.
The trick to this business, as we all should know, is to expect...
How do you know he was signalling to go towards the Princess Royal? He could have been heading to Caxton Way....
Anyway...if he entered that roundabout from Sussex Road, signalling left, as you seem to suggest (wrongly, in my opinion) that he should have done, I'd have expected him to go into...