I’m not particularly sure how it works in the UK, but in the US, the two main ways of showing mastery of a subject to an employer are either having relevant experience in the field (a portfolio of coding projects for a software engineer, or design projects for a mechanical, or just having relevant experience on the resume) or holding a degree from an accredited university.
MIT (and several other higher education schools in the US) offer course materials online for free. The tradeoff, of course, is you don’t get a degree, but as far as teaching yourself the topic, it’s not a bad way to go. You could then work on projects that let you apply that new knowledge, and show those as proof of competency.
Or the good ol’ fake it till you make it, and just lie outright on your resume, banking on the fact that everyone is useless right away, and they’ll teach you what you need to know pretty quickly. (I don’t recommend this, but it is technically an option)
On August 26, 1935, the United Auto Workers established an elite union for all auto mechanics. Its purpose was to teach the lost art of collective bargaining and to ensure that all the union members were the best compensated mechanics in the world.
They succeeded
Today, the UAW calls it a union. The mechanics call it:
TOP WRENCH