Assalam o alaikum
Respected Mufti Saab
A family member of mines is going for Umrah from London, but he wishes to stay for few days in Jeddah before going Makkah, could he wear the Ihraam in Jeddah before he leaves for Umrah.
Await your Urgent Response.
In the name of Allāh, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-raḥmatullāhi wa-barakātuh.
It is compulsory for someone who is going to Saudi Arabia with the intention of performing Umrah to wear the Ihraam and make the intention for Umrah before they pass the Miqaat (the boundaries set for entering in the state of Ihram for Hajj or Umrah). If the Mu’tamir does not wear the Ihraam and enters the Country he will be sinful and liable to give one Dam  (Slaughter a Sheep or Goat) .
Hence in this case it would be better for the family member to wear the Ihram in London and make the intention there also. If there is some hindrance then the member can wear the Ihram on the plane and subsequently make the intention before passing the Miqaat on the plane. Since he has obtained a visa for Umrah this will reflect upon his intention.
And Allah Ta‘ala knows best
Mufti Luqman Hansrot