Before finding this thread, I had planned to buy a Three SIM card from Amazon before traveling from US to England for four weeks. I would want data tethering (for a laptop) much more than voice or text, but would want some voice minutes. I have an unlocked iPhone SE and I have changed SIMs in an iPad, but I'm completely ignorant about registering a SIM or doing anything else necessary before turning on the phone and using it.

Could I get detailed suggestions for a course of action, please? Money is always important, but simplicity is at least equally so. Thanks in advance.