A British Airways Airbus A321-200, registration G-EUXL performing flight BA-1486 from London Heathrow, to Glasgow, was climbing through about FL 200 out of London Heathrow’s runway 27R when the crew declared PAN reporting they were lightheaded. The aircraft levelled off at FL 220 and decided to return to Heathrow for a safe landing on runway 27R about 30 minutes after departure. A replacement Airbus A321-200 registration G-EUXF reached Glasgow with a delay of 3 hours. Great Britain’s Civil Aviation Authority confirmed both flight crew were light headed and after landing reported to emergency services that they both nearly passed out. The passenger oxygen masks were automatically released. The Air Accident Investigation Branch have opened an investigation. A passenger reported that there was a call on public audio from the cockpit requesting a senior cabin crew member to the flight deck, then cabin crew began to rush carrying portable oxygen cylinders and oxygen masks to the cockpit.