Aviation (specifically 'pilot') - a lot of money and training to get your foot on the ladder. You'll break even after 5-10 years and start enjoying your money, which will now be around £60-100k. Very prestigious (even nowadays), lots of responsibility, great people to work with, but fairly poor job security when stood against comparable jobs.
Logistics (specifically 'HGV driver', or 'LGV driver' if we want to be anal about terms) - very low initial investment, qualified in a few weeks/couple of months. Instantly earning £40-65k, lots of jobs with decent unionisation (Royal Mail as mentioned above - fuel tankers can be lumped in this category too), option to work through an agency and work when you like, very chilled job most of the time, and you truly do feel like your own boss. However, you will be spoken to like garbage and treated as sub-human on an almost daily basis, by managers/customers/colleagues, in the majority of jobs. You will also be working between 40-70 hours a week to achieve the above figures.
I've held both the job titles above in my life, and I'm joining the railway shortly, so it's impossible for me to draw comparisons in this respect yet. I do know a former tanker driver who has moved into train driving, and upon reflection, they vehemently stated to me that they would never return to logistics, generally for the reasons I mentioned above. Make of that what you will. Funnily enough, I also know a pilot who became a HGV driver and stated the same thing. Horses for courses.
I do get the impression that leaving the railway shouldn't be done lightly, for all the reasons people have listed above.