If the aircraft is an SEP Class variant which does NOT include items for which formal differences training is specified in the Aircrew Regulation or UK ANO (e.g. a Cessna 152 pilot who wants to be 'checked out' on a Cherokee), then variant familiarisation may be given by another pilot who is familiar with the variant in question.
If 'differences training' is mandated for the new variant (e.g. a Warrior pilot who wants to be 'checked out' on a Turbo Arrow), then differences training must be given by an instructor qualified to fly that variant.