I'm a Sydney bloke who spent 3 years living in Perth, and I found the contrast between the two cities remarkable. Sydney is built mainly on rock and alluvial soils, Perth is rock & sand.
Hence gardens and lawns sustain in completely contrasting environments, dictating the look of the place. Perth more 'native' - reds and yellows, Sydney more 'greens and browns'. Perths sits on the Swan river - Sydney on the big harbour.
Rest assured both have beautiful beaches, Sydney's in smaller expanses, Perth's in seemingly endless stretches.
On return there two years ago on business, I was staggered by the rate of growth - Perth is stripping the other state capitals.
What I really enjoyed about Perth was the facilities for sports and entertainment in seemingly every neighbourhood - there is literally, something for everyone. Sydney does have these facilities as well - obviously - however less lberally sprinkled, and there's more folks using them.
Perth has a huge UK expat population, you won't be lonely there!
One of the newer options is for people to purchase coastal properties in Rockingham to the south, and Yanchep etc. to the north - all of the burgeoning beach suburbs are connected to good roads and public transport, and, importantly - good schools.
Every cuisine under the sun is available, and if you enjoy seafood, you very much have the option of going out and hunting your own!
It's the move of your lifetime, and your family's - and one you won't regret. Good Luck.