I have an 18 month old male King Charles who has gradually become extremely possessive and impatient.Either my partner or I usually have to stay at home for family reasons when the other goes out, and so the dog is never left home alone. When one of us does go out though, even though the dog knows he will not be left alone, he barks frantically for some time. The same thing happens if the three of us are out together and my partner or I go in a shop, he barks then. It seems that he wants us both with him all the time.He's also become very impatient. If we have to wait to cross a road he now barks furiously and taking him on a bus has become a nightmare because he can bark throughout the journey, even during a fairly short one. This behaviour has developed since he was about six months old and is gradually getting worse. Should I consult a dog behaviourist?

This is quite unusual behaviour for a King Charles Spaniel, who are a lot quieter than their cousins, the Cavalier King Charles Spaniel, who are much more vocal & outgoing.

It is impossible to diagnose/interpret behaviour without seeing the dog & behaviour in the flesh.

What you describe is not possessive behaviour. I would suggest you contact a reward based behaviourist for an assessment.

Have you ever done any training with your dog

I would get help, yes.  The first thing you describe sounds like separation anxiety, rather than guarding behaviour, and that needs a very particular approach to solve it.  The other may simply be poor frustration tolerance but it's harder to say without seeing it.  So in-person help would be a good idea, I think, especially as it's now been going on for a year and is getting worse.

Make sure you find a force-free behaviourist who does not talk about dominance, pack leadership etc, as these are outdated, disproved ideas that won't help you.  Try the APBC or IMDT to find a good person.