I was in a similar position to yourself last year and a few months back started as a Trainee Train Driver, I'm mid-twenties but with no real commitments. That said, I do regret not getting the fully funded cadet programme I applied for last year, it was the best scheme in Europe with one of the last airlines which offers really good T&Cs and I came quite close. They should be recruiting again later this year, they fly Airbus types with a plant motif on the tail. So, apply for that when it opens.
The trains can be quite difficult to get into and indeed there is a broad British railway equivalent to PPRuNe which exists and offers plenty of advice, you may have come across it already. You are only allowed to fail the industry-wide aptitude tests once and if you have a second fail you cannot apply again for any driving position with a UK train company. If you pass, the tests are valid for up to five years and you can carry the tests from one company to another. The one thing I will say about the railway is beware of Driver Only Operation (DOO), if you're DOO, you'll be dispatching your own train and if you kill/injure someone in the process the ramifications can go far beyond job loss (see James Street and Hayes and Harlington).
The good news on the railway is that your training will be paid for and you'll be paid £20-35k during training, with a typical basic of £50k once qualified for a 35 hour four day week. If you fail training on ability but have a good attitude, they'll likely offer you a different role in the company. Train companies often recruit much older folk too, quite a few 50+ get in as Trainee Drivers.
On the airline side, as you'll have seen on here, there's the mentored programmes (most are integrated as opposed to modular, some involve parting with more cash than others, a few airlines will guarantee a loan, most won't) and non-mentored. For non-mentored, see the Integrated vs Modular thread.