[{"countries":["UK","New Zealand"]},{"countries":["USA","UK"]},{"countries":["UK","USA","France"]},{"countries":["UK","USA"]},{"countries":["USA","UK"]},{"countries":["UK","Italy"]},{"countries":["Italy","USA"]},{"countries":["New Zealand","France"]},{"countries":["UK"]}]

db.collection.aggregate([{$match:{countries:"UK"}},{$unwind:"$countries"},{$match:{countries:{$ne:"UK"}}},{$group:{_id:"$countries",count:{$sum:1}}}])