Flown thru most major UK airports and never had any problems. The current state of affairs means that Sporran, Belt and kilt pin would need to be removed. If you need to get double checked it will be with a hand scanner.

Some airports get touchy re the kilt pin especially if it has a long pin. My advice put it in your case or don't wear it.

Dirk's are of course a no no

The sporran/bag thing is a red herring nothing to worry about.

enjoy your trip