From an MOT point of view if the wheel arches are bolted, screwed pop riveted, etc to the bulkhead then they are not an MOT testable item, UNLESS there is something attached to the wing that a vehicle relies on for it's safety. So if you have the ABS module or similar fastened to the wing then that wing would be liable to be tested by the MOT tester. If the wing is welded to the rest of the bodywork/bulkhead then it is a MOT testable item.