この - this
アプリ - app (application の略)
改善 - improvement
余地 - room
あります - there is / there are
このアプリは改善の余地があります - This app still has room for improvement もしくは This app can still be improved (upon).
still - まだ
改善の余地がまだ多大にある - There's still lots of room for improvement / There's still plenty of room for improvement
イギリス 
