I think you can sometime get to use these if you have bank accounts that have extra's attached and you book your holidays through them. Accounts like RBS Royalties Gold Account I think let you use these facilities. then again you usually have an admin fee to pay for these accounts
You can literally pay for access, one of the ways being online (google the airport lounges or specific airport). There are different ones and the ones for specific airlines are generally not open to payers "off the street" but the non-specific ones are. The going rate is around £20-£30