I believe you have to do an LPC on the type and I am not sure about the medical. If you go to the SRG site and download SRG1118 it gives instructions.
But in a nutshell, you have to prove their is no UK licence holder available, that the job is only a temporary job and it is restricted to the specific type. The validation is issued only once after that you must convert.
You will need an RT Licence, Class 1 Medical and your logbook to go with the submission and the operating company who are sponsering your visa would normally endorse the application.
If you are a UK/EU resident then none of the above applies, you need convert!!