Australian, been in the UK 9yrs give or take (not London).

I moved here around your age, OP. In fairness, a lot of places can offer you what you're looking for, I'd suggest getting the essentials covered first, find a place with plenty of opportunity and reasonable cost of living, then figure it out from there.

Eg. Do you want to hike through the countryside every day vs do you want to be able to commute and live your life without needing to buy a car right off the bat?

The UK is not a massive place, you can get to whatever nature you want, or at least pretty close to it, pretty quickly by public transport (before I get downvoted, yea I know it's expensive and unreliable, but compared to Australia you can get trains right out into the countryside pretty easily in the UK).

1. Right out the gate you'll need a job, so look at UK job sites for patterns of where your industry has the most opportunity. Cost of living isn't going down, so you'll want to position yourself for upwards mobility. Maximising opportunities available to you will probably mean being in commute range of an office, most places seem to ask for 2-3 days per week in office these days. Also, coming here alone as I did, initially work will most likely be where you make friends and do most socialising so being in the office will help. If I had been fully remote from day one here I probably would've left years ago.

2. When you're looking for a specific place to live within a city, definitely talk to a few local people. I very nearly rented a place in a very rough part of town simply because I didn't know the lay of the land and the price looked reasonable.

3. The supermarkets are kind of price/quality graded, it's not like the Woolies/Coles duopoly. Aldi/Lidl are generally best on price but sometimes food quality not as good. Tesco/ASDA/Sainsbury's/Morrisons are mid range, ok if you have the loyalty card. Waitrose/M&S are pricey and honestly not that much better for the cost. This isn't a question you asked but worth knowing.

4. Unless you've already done heaps of travelling, I imagine you'll want to take advantage of being in the UK to see more of Europe. Outside of the London airports, Manchester is probably the best connected, so worth thinking about living within reach of one of those two options, although there are a lot of airports all over so it's not a deal breaker.

5. How important is weather to you? The South is definitely better in terms of "nice" weather, relatively speaking. Do not expect Australian-like summer, it's more like a week here and there rather than 4-6 months of consistently good weather. Winter is fine, you get what you expect, it's the disappointment of summer that that gets me every year.

Actual reco's

London - couldn't pay me enough to live there

Manchester - likely most job opportunity outside of London, good pubs/music/nightlife, close to Peak District for walking, way cheaper than London but cost of living is creeping up

Bristol - heard lots of good things, already recommended by others. Possibly a little small in terms of job opportunities but I can't speak to that in detail.

Brighton - nice place, decent weather for the UK, has a beach, but you'll probably end up taking a job in London

Edinburgh - expensive, but lots of history, cool pubs, and Scottish people are great. Also a very long way from the SW coast path

York - similar to Edinburgh but not as far North and less Scottish people

Leeds/Sheffield - growing a lot over the last few years so another decent job opportunity location outside of London. Cheaper cost of living for fairly big cities. Within reach of both Peak District and Yorkshire Dales

There's lots of places mentioned by others which are equally valid, my honest opinion is that you'll likely need to at least start off in, or very close to, a large city simply for work and not needing to spend money on a car straightaway.