You asked if a national of Saudi Arabia needs a visa to come to the UK in transit.
At the discretion of a UK Immigration Officer, you may transit without visa (TWOV), but only if:
you will continue your onward journey from the United Kingdom by air within 24 hours, and
you will hold a confirmed onward ticket, and
you will have the necessary documents (passport, visa, etc. ) for the country to which you are travelling and any countries that you will pass through en route, or
you will arrive on a cruise ship and intend to depart on the same ship within 24 hours.
If not, you need a Visitor in Transit or Visit visa.
http://www.ukvisas.gov.uk/servlet/UKVisasDoINeedAVisa?url=%2Fservlet%2FFront%3Fpagename%3DOpenMarket%2FXcelerate%2FShowPage%26c%3DPage%26cid%3D1006977149962&purpose=Transit&nationality=Saudi+Arabia&location=Saudi+Arabia

السلام عليكم
لا اخوى مايحتاج فيزا لانك بتنزل ترانزيت لندن ومعروف ان الترانزيت مايحتاج فيزا الا اذا كنت ناوى تطلع من المطار وتنزل لندن

Boo3lam
انا اتفق مع اخوي
لازم تتاكد ان المطار اللى راح تنزل فيه في لندن هو نفس المطار اللى تاخذ طيارتك للمغادرة
لان في لندن خمسة مطارات دولية