OK Lotus Eater, I'm a peaceful and reasonable person, but you asked for this.
If you're in the UK and can't find Vegemite, go to a major supermarket, find the Marmite, and next to it you'll see the Supermarket's less-expensive own-brand version. That, I promise you, will taste a lot more like Vegemite than Marmite does. The Marmite people have a secret process which gives it that unique and magisterial depth of flavour.
Interestingly though, I have the spellcheck on as I write this, and it accepts Vegemite but not Marmite. Hmmm...