Theres loads of reasons why staying in the uk is just as good an option.
You wont need to spend all that money shipping your stuff over.
You wont need to pack up your life while trying to look after your dc.
You know the education system is better here and you know how it works.
You know you wont get 3rd degree burns taking the rubbish out like in Oz.
You know theres a working time directive in the uk and you wont be asked to work 6 days a week 12 hours a day (my cousin does this unfortunately).
You are entitled to minimum holidays in the uk. This often isnt the case in Australia.
You wont need to make new friends if you stay.
You dont know what will happen or how much time you have to deal with it if your child is bitten/stung by something in Australia (evil creatures live there).
You know what the food tastes like here and how much it should cost. (A loaf of bread is $7 at my Nans local store in Australia so no sandwiches for her anymore).
You understand how things work in the uk, Australia can be a minefield if you dont know all the right laws and restrictions.
Honestly theres loads of reasons why staying in the uk is fine and like you have already said you have a nice house and nice life. You will pay your mortgage off in a few years and your children are happy and healthy.
And the uk has history, proper pubs, real food, sense of humour, countryside thats safe to visit, real distinct seasons, more tolerance (personal experience with that), coffee shops that serve proper tea/coffee, and the best bit for me is less boob sweat in the uk it goes on for months in Australia 
I went expecting great things but returned fairly quickly (18 month). Its just not my cup of tea. Its too hot, too expensive and you have to work too many hours just to make ends meet.