II give them a friendly, non-aggressive tut-tut to delicately remind them to F the F off. Works 100% of the time, you should do it too. I used to indicate left and just wait for them to fall back or pull ahead but they never notice which made me realize they actually have 0 awareness/attention but the horn I’ve been doing for a month (admittedly doesn’t happen that often) never fails to wake them up. I wish I could do it to the jolly fellas who like to do the same on the O/S but technically it’s not forbidden though eventually they’ll get swept by a left hander and have the motorway shut for 2 hours.