One of the best ways to make a difference to the financial world is to join your local credit union. A credit union is a bank owned and operated by local people, and not operated for profit. It’s complicated to set one up, so try to join one that already exists, nearly all areas have one, formed by a common bond (usually location but also there are ones for professions and religions).
Rather than interest, you are encouraged to save (not get into debt). You get dividends based on savings, and loans are only for those who save regularly, interest set by law to be low (around £50 in interest on a £500 loan over a year) and like banks, they are covered by Financial Compensation services from government up to set amounts.
Joining is simple. Just turn up at the office or pop-up (community shop, school) with £1 and some form of ID (some let homeless people us a hostel address). Small branches stick with that, but larger branches offer visa cards to mortgages to business help. Find your local credit union. There are also credit unions for: