You can. If you're entered for the exams this Summer then you should be able to re-sit, for free, at the same centre this Autumn - although details have yet to be announced.
Next Summer will probably be back to normal. Yes, you can re-sit. Your sixth from may have you back (likely to be the cheapest option) or you may need to find an alternative centre which accepts private candidates - any can,
Whenever you re-sit, your grade is based on all the most recent exams.