If you get your provisional license during that first 12 months, you are not required to show L-plates during that first 12 months (the time that your foreign licence is still valid). You also don't need to be accompanied, and you can drive on motorways (again, just during the first 12 months). After that first 12 months, you must show L-plates and be accompanied, so it's best to start the process as soon as possible.
What will be triggering date of these 12 months? When the residence permit (RP) is issued or when the resident reached the UK (which could be a few weeks or months later than the RP issue date).