Hi - We've just been through this for mostly the same reasons as yourself. We were in Melbourne for 7 years 3 months with our two children being born in Australia. The distance / cost of flying to see family was a concern going forward as was the lack of decent affordable housing in good school zones, anecdotal evidence regarding education standards and diminishing job opportunities. Despite this, making the decision was really tough because Australia is such a beautiful country. The decision was tough because we felt that to stay (in Oz) would deny the grandparents and my brother from knowing and loving our kids, probably never being able to afford a house for the kids (without commuting for hours and in so doing denying them a father) but to leave Oz meant taking the sun away from the kids. Family, education and opportunity are more important than weather to us but that's not the same for everyone and that's okay, we're all different.
Regarding the decision and actually leaving I personally found it helped to trick my mind by telling myself I'd try the return for 2 years in the UK and that many people smarter than me have returned and are happy. I can tell you that it was the right decision to move back. I'd forgotten how beautiful the UK is (even in January). My mind is no longer split by the whole "should we go, the weather's bad and we have no family here, no let's stay it's a beautiful warm day and the kids are running about on the beach, no let's go the schools are no good and we can't afford a house" routine. It grinds you down after a while.
Only been back for a few weeks but it's so good to see the family again. We've just bought a house in a village not far from family with a decent sized garden and have registered with the local primary. A life that we struggled to achieve after living in Melbourne for years came together in a matter of weeks in the UK including a good stable job.
You have citizenship so you can always go back if on returning you think it was a mistake. Yes, there's a financial overhead but at the end of the day you can keep the move costs down if you plan, emigrating is like an expensive holiday. Finally nothing is for ever so don't feel like you're ever making an irreversible decision whether it's coming back to the UK or going to Oz. For me I think that's where the difficulty in making the decision came from, quite literally life is suddenly at a crossroads and both forks lead to different and equally desirable futures. Of course that's not the case, it's instead a change of location for an unspecified amount of time with an option to change tack if it's not working out (although I hope we don't go back to Oz!).
Good luck with decision.