Airline and immigration check the visa in the passport but now we have moved to the evisa they ask to see the printed version. I would not expect them to take my word for it and they do pull people out if they don't have the correct docs
A little tip if your unsure on anything just upload the pic of your passport (I think I did this for at least 3 questions) if they need more details they will contact you, the staff at London embassy are very good.