My understanding is (assuming you have the relevant hours for the rating required) - home study, written examination at any test station, flights with an instructor sufficient to receive a "sign off" that you are capable of passing the test.
For the ATP there is no sign off required - so no instruction required , for the PPL...they will just give you one...no conversion needed.
Seperate tests avaialble for PPL, Multi Engine, Single Engine Commercial, Multi Engine Commercial. Single engine I/R, Multi engine I/R, Single engine ATP, Multi engine ATP.
Conversions available for your UK PPL (nothing to do, other than turn up for an appointment) and your UK I/R (one multi-choice written test) if you want them.