UCAS is the application website for most schools in the UK.
How does UCAS work?
Students can choose five programs on the UCAS Application. This can be five different universities, or several programs at one specific university.
UCAS schools are primarily three year programs (except Scotland) where students concentrate on their major for the entire time. Essentially students going to these schools MUST decide on a major going in and stick with it. There is no changing later.
UCAS has a personal statement which is very different from the US application essays. This statement should be straight forward and specifically talk about what the student wants to study, why, and how they have shown their interest. They should discuss their goals in the field and any prior experience that they have had in it.
Some top universities within the UK like Oxford require interviews and additional tests, depending on the major/program students are applying to. Be sure to thoroughly review the application requirements for the program you want at each school specifically.
If you have your heart set on applying to schools in the UK, having high-scoring AP Exams that you completed is very helpful as the UK school system favors exams like these over SATs/ACTs as there is no equivalent in the UK.