I'll add that even as a frequent flyer with Oman Air, even when I have a valid visa they always check I have a return flight. Obviously all carriers are different, just my experience them them in particular.
, take no notice of the sad twats on here. Get to the airport, make sure you have 20k cash, be prepared to sign some paper, lose your 20k, and be banned for 1 year.
Don't get caught beforehand, it will land you with a 5 year ban.
Sometimes from business class, other times I've managed to get one off of airline crew who remember me. My trips to the UK are usually about 6 or 7 weeks long, but the immigration have a loooong look though my passport, and my heart skips a beat everytime.