Can I put a UK 3 pin plug or use an adapter on a dishwasher I brought in South Africa?
Standard electrical outlets in both Britain and South Africa supply 230V at 50Hz. There's a fair chance your dishwasher would adjust and work on 120V/60Hz power, too if you were bring it to someplace like the U.S. or Canada. That's not guaranteed, but manufacturers tend to like to sell their goods into more than one country.

Regardless, electricity in Britain is just like the electricity in South Africa, so if you can find the right plug adapter (type M to type G) you'll be fine.