Politics is all backroom stuff and old boy networks. You may get involved, but to even start influencing a little you will need many years of networking before you can ever get started. And statistically speaking, you are unlikely to succeed. Objectively it is the only advice that makes sense, but viewed from the individual it doesn't really yield any kind of solution.

If you organise yourself you can make an impact a group of 50-100 is easy to build and can achieve a lot. Once you build a group it's easier to network as well. This takes a lot of time and effort for 5-10 years. Don't defeat your voice, fight cynicism.

There is a certain amount of agency in UK parliamentary selections, but the leaderships of both major parties have (for a good while) been keen to limit that. I'm not saying that it's futile -- you'll certainly get a choice of candidates that the leaderships are happy with -- but I'm not sure it's a decent mechanism for achieving change anywhere except at the margins.