I am going to be flamed here, but I think that allowing a child of "Timmy's" age to interupt is the right thing to do, and I certainly let them do it at the age when it was a question about "look I can clap my hand". Now that they are teenagers, absolutely I expect them to wait, and not interrupt, but teenagers are very different from young children.
From Timmy's perspective - I am his mother, the only one he has got, the only one who is capable of giving him the reassurance and support he needs to investigate the wide world without fear. My friends, no matter how serious their issues, are adults who can get their support from multiple sources, or if they want it specifically from me, can find me at a time when Timmy is not around. If Timmy comes to me wanting excitedly to show me he can clap, and I tell him to wait, I am effectively telling him a) his achievement is not much of an achievement and/or b) my "grown up" friend is more important than he is. Neither is a message that I want to give.
Timmy will learn not to interupt from interacting with his peers, and with his teachers at school (who come down on that sort of thing like a tonne of bricks). At the point that Timmy starts seeing me more as a friend and advisor, rather than a mother, it is also the time for him to start respecting my boundaries, and not interupting me - and this generally happens once they hit adolescence. They start expecting you not to interrupt them when they are having important social calls, and you need to demand that they to do likewise. Prior to that, I believe that his relationship with his mother is special, and I want him to feel that as well, that he can trust that his mother will be there for him, and will not prioritise her friends over him. Usually it can be done in a very quick way, so as to get back to the conversation, but sometimes it can't (depending on what Timmy's needs are).
I also expect that Timmy at that age will stop his conversations with his friends when I tell him that it is hometime, and not ask me to put my hand on his thigh and wait until he has decided that he is done, or his friend stops talking or playing. I am his mother, and I decide when it is time to go home. I may try and build in warning, but I would be horrified if he turned around to me and said "Mummy you are interupting, wait until there is a break in the conversation", and then went on playing. But I cannot realistically expect him to drop everything and listen to me, if I don't model that behaviour. With my teenagers, I don't expect them to drop everything and listen to me, and I expect them to give me the same courtesy. Likewise, I totally expect my friends to prioritise the needs of their own children over me, when those children are small, and I would be horrified if they did not. My stories, no matter how serious, should not get between a mother and her child, and if I needed serious support from a mother of a small child, I would make sure to seek that support at a time when the child was not around (at nursery, school, after bedtime etc).