I'm an Australian living in London, I've lived here for 12.5 years. I can answer you question from the London perspective:
Lifestyle - this is in my opinion the best part of London. You will never be bored as there is something for everyone here: museums, theatre, parks, history (old and modern), galleries, architecture, music of every kind, festivals, markets and shopping, food from most corners of the world, I could go on and on. Some attractions are free, the west end shows are often reasonably priced.
Standard of living - London housing is very expensive. Public transport is also expensive. Don't even think about a car. My recommendation is to travel by bicycle. Accept the fact you will be living in a tiny shoebox apartment with other people and life becomes a little easier.
Weather - this is a temperate climate meaning you will have 4 distinct seasons. It is also affected by the Atlantic jet stream so the seasons can be changeable. The British have a saying "there is no such thing as bad weather, just the wrong clothes." For the most part this is true, British weather doesn't always deserve such a bad reputation. Having said that, if you are a true sun-lover you will hate Britain. But if you can put up with grey days then you will discover autumn trees, snowflakes in winter, spring daffodils and English roses in bloom.
Disposable income - if depends on what happens to the £ sterling, in the past you could count on the fact an income in the sterling means you buy you cheap holidays abroad. Now the value of the pound is sliding and this is less true. If you think you might stay a while you may have an opportunity to choose a career path that will allow you to specialise in a way you couldn't in Australia, meaning your earning potential could be better in London. Mostly it will depend on how competitive your Industry is and if there is a shortage of your skills/experience in Britain. Do some research and it might become clear.
Ability to travel - being based in the UK is simply the best for travel! The UK regions and Ireland have so much to offer, Europe is on your doorstep, North Africa and the Middle East are also possibilities (when it's safe 
).
Other things to consider:
Rest of the UK - Britain is so much more than just London. The capital's prices can be eye watering so seriously consider Edinburgh, Manchester, York, Leeds and Bristol. All are wonderful cities, most have a healthy student population for a fun vibe. If you want the London experience on weekends and a decent sized home then consider Birmingham.
Cost of visas - if you intend to stay the full 5 years to qualify for residency and citizenship (1 extra year) then research the application fees and factor that in so it doesn't come as a shock. The permanent resident application fee is steep. If you decide to apply for it there is also the cost of sitting the "Life in the UK" exam (yes it is a thing you need to study for!), any additional references and above all keep records of your travel outside the UK because you must give a detailed account of absences of 2 days or more. If you travel too much you may not qualify.
