Re the spiders, they're really not that bad, I have had much bigger spider issues here in the UK than I ever did in Oz. I'm sure there are more spiders here than in Oz. You'll probably find the cockroaches worse, especially on the coast, make sure you get some cockroach baits in that first shop
Winters are not as cold but the biggest difference in they are much much much shorter - really just a couple of months of blurgh weather. July (winter) on the Central Coast can be Tshirt weather. It rains though. You get more rainfall in Sydney than London, because when it rains, it really rains, but it may rain solidly for two weeks but could be followed by months of blue sky sunshine. None of this day after day after day of grey sky drissly awfulness! You're not likely to have central heating so I'm sure you'll want to buy a heater.
In my experience, some things are cheaper, some more expensive, and it really just depends on what you value and how you like to spend your time and money. You definitely won't find castles, art galleries and museums or M&S ready meals but you should be able to find a really nice quality of life that isn't so easily found over here. But, it all depends on what you personally value, which is different for everyone. I would say there are more opportunities here but your quality of life is better there.
You are right to say that you will have to work, I think many people think you can just sell up your average home in the UK, buy a mansion at Bondi Beach and never work again, but you should just find everything just a bit easier
Cars are more expensive, petrol is a lot cheaper. Food probably compares but there's more free stuff to do because the weather is better. Property is expensive but I think you get more for your money. BB and telephone will be cheaper and better here in the UK along with TV, but with any luck you won't need 500 Sky TV channels as you'll be outside BBQing with your new friends instead
Have you thought about renting your property here rather than selling? Easier to come back if it's not your thing.
Will your kids get bored of the beach? - again, depends on what they like to do but I grew up in Sydney and had a fabulous childhood with only fond memories and was never bored. Can't comment on the schools as it was some time back that I went to school and any experience wouldn't be relevant now.
Good luck
