Hi Sophie, congratulations on your pregnancy! I am also due in October and have a 3 year old
My partner is Australian and although his family live in another state, I still feel like we have family around. My parents also visit from the UK every year for 3 months which is amazing. If I didn't have this I would really struggle with being here, but I love our life here in Melbourne for now. I would also have to move to my childhood area for the level of support many have, and I don't think our quality of life would be great. Another area of the UK - absolutely! But we would be back to that position of having to build up a new network, which has taken so long to establish here.
I say go - you have citizenship and there is absolutely nothing wrong with the UK! Many of my friends have made the move back after having kids and settled well, one has bought her dream home where she really wouldn't have had the opportunity here.
I would be prepared for feeling unsettled on returning for a period, I hear it's quite common, like a reverse culture shock! But I truly believe that's just what happens when you've been an expat - you'll always leave a part of you in both Countries especially if you have experienced so much happiness there.