How much more money? I'm frankly not sure there is any skill you can learn on your own (without full-time commitment and/or paid formal education) in a reasonable time that would make financial sense for a typical 50k/year office clerk to learn, at least not in the UK.
The closest to this would be software engineering, but even then, it would take you ~10 years of experience to reach the level of a proper senior software engineer, and even that only gets you 80k on average in the UK - a sizeable chunk of which will get stolen by the taxman.
10 years of effort starting from scratch to not even be able to double your salary (and again, the net salary is even smaller, as taxes increase) is not a winning strategy. You're better off bullshitting and playing office politics to move up in the pecking order at your current company and increase your salary that way.