As rupert says, some just want to travel, others aren't that fussed. You're 24 and your parents should understand that it is just something you want to do. My parents aren't big travellers and couldn't grasp why I wanted to sod off to Australia on my own, but they supported me and though your parents will miss you, they will be happy knowing you are doing what you want to do. Plus... staying in one place forever.... Boring! Seeing the world doing new things, different jobs and generally cool stuff.... Awesome!
A good majority of people travel alone and there is a consensus among solo travellers and groups alike that travelling alone is a character building feat, and you have the freedom to really do whatever you want. 2 years in Oz, NZ and SEAsia, I would be extremely hesitant to ever travel with others as being alone is really just amazing. You area never actually alone!
Do Sydney-Cairns on the Greyhound. You have a good job back in the UK - there's lots of work if you know where to look and sell yourself well. It's great fun and you will meet lots of alike people along the way of all ages from all over. You said it - you can always head home if things don't work out... but if you have that need to do it... just do it! It might be the best thing you ever did.