I think these questions present a very interesting dilemma. They are guaranteed to get asked, tons of them and often. But they are of little use to most visitors aside from the question author.
And either most or least importantly, depending on your politics, is the simple fact that while expats may desire foods or other things from home, making it easy to get those things by staring at a website is actively harmful to the expat experience. Being an expat can be about much more than eating your favorite cheese from home while working in China. If you focus on keeping things the same when they are clearly not the same, you may limit the depth of your experience in many ways. It's daunting to re-learn all the brands and products and shops and conventions, but isn't it great when you've done so?
With that out of the way, I'll take a crack at your specific examples:
- Buying Australian Food in the UK?
- Buying Australian Food in London?
- Buying Australian Food outside of Australia?
Too vague, nobody can be sure what "Australian food" is, and it will end up being a popularity contest among foods that answer author A enjoys vs author B vs C.
- Buying Vegemite in the UK?
- Buying Vegemite in London?
- Buying Vegemite in Southwark, London?
Too specific, and London is hardly uncharted territory. A web search should get you started, or you can just pop over to Marks & Spencer, which you hardly need a Q&A website to tell you about.
- Strategies for finding foods from home when you're abroad?
There can't be a right answer. From what I know of other StackExchange sites, this would be a non-starter, as it is a sort of list or poll.
- UK customs rules on bringing a few jars of Vegemite into the UK?
Customs rules, now we're talking! This could have specific answers, which could address much more than Vegemite (canned spreads or so), and might not be immediately clear from a web search (I haven't checked this specific one, but similar ones definitely aren't always clear). That said, there's no reason to have this on Expats instead of Travel--an existing site which if they'd take this question, would be a better fit as it would have a wider audience there.